Configure project :
This mapping 'snapshot_20171003' was designed for MC 1.12! Use at your own peril.
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ForgeGradle 2.3-SNAPSHOT-d5c1380
https://github.com/MinecraftForge/ForgeGradle
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Powered by MCP
http://modcoderpack.com
by: Searge, ProfMobius, R4wk, ZeuX
Fesh0r, IngisKahn, bspkrs, LexManos
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Found AccessTransformer: betterportals_at.cfg

Task :compileApiKotlin FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':compileApiKotlin'.

Could not resolve all files for configuration ':forgeGradleMc'.
Could not find net.minecraftforge:forgeBin:1.12.2-14.23.5.2838-PROJECT(BetterPortals).
Searched in the following locations:
- https://files.minecraftforge.net/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://files.minecraftforge.net/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- https://repo.maven.apache.org/maven2/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://repo.maven.apache.org/maven2/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- https://libraries.minecraft.net/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://libraries.minecraft.net/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- https://jcenter.bintray.com/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://jcenter.bintray.com/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- http://repo.spongepowered.org/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- http://repo.spongepowered.org/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- https://minecraft.curseforge.com/api/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://minecraft.curseforge.com/api/maven/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar
- https://maven.shadowfacts.net/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).pom
- https://maven.shadowfacts.net/net/minecraftforge/forgeBin/1.12.2-14.23.5.2838-PROJECT(BetterPortals)/forgeBin-1.12.2-14.23.5.2838-PROJECT(BetterPortals).jar

You need to run ./gradlew setupDecompWorkspace once before building because ForgeGradle.

thanks