Versions

• Java 10.0.1
• Spigot 1.12.2-R01
• IntelliJ IDE
• Gradle 4.7 (Supports Java 10)

Other plugins in use (With ProtocolLib) for my plugin project

• Vault
• SaneEconomy
• WorldGaurd
• WorldEdit

Use intent
Wanted to start using ProtocolAPI for my plugin to modify/ create packets.

Problem
After following instructions to setting up the gradle.build Gradle kept on failing to import (error on title).

Steps taken after problem
First thing I did was try it with ProtocolLib instead of ProtocolLib-API. Still did not work.

After some digging, I found some interesting stuff. I looked up the repo link and found that there is no such directory of "com.comphenix.protocol" in the "public" database folder, but instead in the "releases" database.

I also realised changing the URL to "http://repo.dmulloy2.net/nexus/repository/releases/" will still not work probably because that is not how Maven/ Gradle repos work (Im guessing its simply not just a folder with directories but more complicated)

In the end, I decided to just import it locally by downloading v4.3.0, putting it in a "libs" folder and importing using Gradle through "compile fileTree(dir: 'libs', include: '*.jar')"

build.gradle settings
(The one that works for now by commenting the line importing ProtocolLib by the maven URL for it)

apply plugin: 'java'
apply plugin: 'maven'

group = 'io.github.flintintoe'
version = '0.1.0'

description = "A Minecraft Spigot plugin for customizable sidebars"

sourceSets.main.resources.srcDirs = [ "src/main/java" ]
sourceSets.main.resources.includes = [ "**/*.yml" ]

compileJava {
    sourceCompatibility = '1.10'
    targetCompatibility = '1.10'
}

repositories {
    maven { url = "https://hub.spigotmc.org/nexus/content/repositories/snapshots/" }
    maven { url = 'https://oss.sonatype.org/content/repositories/snapshots' }
    maven { url = "http://nexus.hc.to/content/repositories/pub_releases" }
    maven { url = "http://maven.sk89q.com/repo/" }
    maven { url = "http://repo.maven.apache.org/maven2" }
    //maven { url = "http://repo.dmulloy2.net/nexus/repository/releases/"}
    mavenCentral()
}

dependencies {
    compile group: "com.sk89q.worldguard", name: "worldguard-legacy", version: "6.2";
    compile group: "org.spigotmc", name: "spigot-api", version: "1.12.2-R0.1-SNAPSHOT"
    //compile group: "org.bukkit", name: "bukkit", version: "1.12.2-R0.1-SNAPSHOT"
    compile group: "net.milkbowl.vault", name: "VaultAPI", version: "1.6"
//    compile group: "com.comphenix.protocol", name: "ProtocolLib-API", version: "4.3.0"
    compile fileTree(dir: 'libs', include: '*.jar')
    testCompile group: 'junit', name: 'junit', version: '4.12'
}

aaah, this I can work with.
it looks like that the stable 4.3.0 refers to the snapshot parent pom v4
adding the snapshot repo to your build script should fix it
http://repo.dmulloy2.net/nexus/repository/snapshots/

@dmulloy2 this really shouldn't be the case, do you make changes to the parent pom often? it might be a good idea to release a version of the parent project into the releases repo (or just give in an use snapshot for everything :D)

Put in the url "http://repo.dmulloy2.net/nexus/repository/snapshots/" and got this error.
I think I should just download it and put it in my libs folder where I can import though Gradle locally.

* What went wrong:
Could not resolve all files for configuration ':compileClasspath'.
> Could not resolve com.comphenix.protocol:ProtocolLib-API:4.3.0.
  Required by:
      project :
   > Could not resolve com.comphenix.protocol:ProtocolLib-API:4.3.0.
      > inconsistent module metadata found. Descriptor: com.comphenix.protocol:ProtocolLib-API:${minorVersion} Errors: bad version: expected='4.3.0' found='${minorVersion}'
   > Could not resolve com.comphenix.protocol:ProtocolLib-API:4.3.0.
      > Could not get resource 'http://repo.dmulloy2.net/nexus/repository/snapshots/com/comphenix/protocol/ProtocolLib-API/4.3.0/ProtocolLib-API-4.3.0.pom'.
         > Could not HEAD 'http://repo.dmulloy2.net/nexus/repository/snapshots/com/comphenix/protocol/ProtocolLib-API/4.3.0/ProtocolLib-API-4.3.0.pom'. Received status code 400 from server: Repository version policy: SNAPSHOT does not allow version: 4.3.0

the url to the 4.3.0 artifact is http://repo.dmulloy2.net/nexus/repository/releases/com/comphenix/protocol/ProtocolLib-API/4.3.0/ProtocolLib-API-4.3.0.jar
that means that your build file looks good to me. maven repos are just some folders, but its not browseable in nexus 3.
I would suggest running with the whatever gradle uses to refresh dependencies and see if it attempts to resolve the jar from dmulloys repo.

Found something more:

From what I could make of it, it looks like parent directory "com.comphenix.protocol:ProtocolLib-Parent:v4-SNAPSHOT" is missing. *(shouldn't say missing, but a 400 bad request error)

> Could not resolve com.comphenix.protocol:ProtocolLib-API:4.3.0.
  Required by:
      project :
   > Could not resolve com.comphenix.protocol:ProtocolLib-API:4.3.0.
      > Could not parse POM http://repo.dmulloy2.net/nexus/repository/releases/com/comphenix/protocol/ProtocolLib-API/4.3.0/ProtocolLib-API-4.3.0.pom
         > Could not resolve com.comphenix.protocol:ProtocolLib-Parent:v4-SNAPSHOT.
            > Could not resolve com.comphenix.protocol:ProtocolLib-Parent:v4-SNAPSHOT.
               > Unable to load Maven meta-data from http://repo.dmulloy2.net/nexus/repository/releases/com/comphenix/protocol/ProtocolLib-Parent/v4-SNAPSHOT/maven-metadata.xml.
                  > Could not get resource 'http://repo.dmulloy2.net/nexus/repository/releases/com/comphenix/protocol/ProtocolLib-Parent/v4-SNAPSHOT/maven-metadata.xml'.
                     > Could not GET 'http://repo.dmulloy2.net/nexus/repository/releases/com/comphenix/protocol/ProtocolLib-Parent/v4-SNAPSHOT/maven-metadata.xml'. Received status code 400 from server: Repository version policy: RELEASE does not allow metadata in path: com/comphenix/protocol/ProtocolLib-Parent/v4-SNAPSHOT/maven-metadata.xml

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

The parent usually only changes with Spigot versions. One will get added to the releases repo after 4.4.0, in the next couple of days

If so, then ill be importing locally until the parent gets added. Case closed!