Question
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How can i compile this mod? i dont have package com.supermartijn642.core

In the build.gradle file, swap the current core library line with the one that's commented out. It should look like this:

- // implementation fg.deobf("curse.maven:supermartijn642s-core-lib-454372:${project.core_library_file}")
- implementation fg.deobf("com.supermartijn642:supermartijn642corelib:1.1.0-forge-mc1.15")
+ implementation fg.deobf("curse.maven:supermartijn642s-core-lib-454372:${project.core_library_file}")
+ // implementation fg.deobf("com.supermartijn642:supermartijn642corelib:1.1.0-forge-mc1.15")

Then in the gradle.properties file, replace the core_library_file property with the id for the latest file on CurseForge for whichever Minecraft version you're using here: https://www.curseforge.com/minecraft/mc-mods/supermartijn642s-core-lib/files

That should do the trick 🙂