need 1.21.1 very much
Can you tell me how to find or make it

download this branch, extract, open powershell then type .\gradlew build

I use the meteor client fork for 1.21.1, you can find it on their website.

In the 1.21.1 branch you can find the SETUP.md file that explains how to compile the Mod.

In my case the forge version wouldn't compile and I only wanted the fabric version so I added

task buildFabric {
    dependsOn ':fabric:build'
}

build.dependsOn buildFabric

at the bottom of the build.gradle file in the root directory. IntelliJ then allowed me to just click the little green build icon left next to the line containing the code you just pasted.

Whis worked for me. If you need the forge variant maybe (I did not test this) try replacing the ':fabric:build' with ':forge:build'