Is 1.12.2 not supported anymore? It doesn't get imported into Eclipse at all, but later versions can. No error is given.

My advice to you, use Intelij IDEA. 
 

User Interface: IntelliJ IDEA has a more modern and sleek user interface compared to Eclipse, making it easier to navigate and use. The overall user experience is generally considered to be better in IntelliJ IDEA.

Performance: IntelliJ IDEA is known for better performance and efficiency. It handles large projects more smoothly compared to Eclipse.

Code Navigation and Refactoring: IntelliJ IDEA's capabilities in code navigation and refactoring are quite superior. This can be very helpful when working with complex Minecraft mods.

Gradle Support: Both IDEs have good Gradle support, which is essential for Minecraft Forge modding. However, some users find IntelliJ IDEA's integration more intuitive and reliable.

Edited July 6, 20232 yr by BaKY
(more info)

The version you are using is no longer supported on this forum.

Please update to a modern version of Minecraft to receive support.