A problem occurred evaluating settings ‘forge-1.20.1-47.2.20-mdk’.
Cannot invoke method maven() on null object
If you expect any help, you should provide more than close to no information.
What versions are you using (Gradle, Java, ForgeGradle, …)?
How does your build script look like?
What is the full error?
What do you get when following the advices the build gave you, especially the --stacktrace output, or even better the build --scan?
…?