Plugins don't work MCreator 2021.1

Thats strange, did you try put them directly in the plugin folder? 

You have to use plugins compatibile with these versions. If you used latest Fabric plugin, it works with 2021.2, not 2021.1.
If you want to find version compatibile with 2021.1, you should use its official GitHub release history.
The latest version working on 2021.1 is 1.16.5 - 1.3 - Structures and Particles (14 August 2021), but as Goldorion wrote in changelog, next versions have huge changes that may cause issues.

Therefore I'd recommend using 2021.2 and latest version of the plugin, since not only it has much more features, but it is also much more reliable.
You can use several versions of MCreator if you unzip them instead of installing, so you can keep both 2021.1 (for 1.12.2 plugin) and 2021.2 (for Fabric plugin).