Some parts are defined as non-overridable. Simply put, they should only be defined once. An example of this is the schema part -
part:@sanity/base/schema, usually defined as the first thing in your studios
It doesn't make sense for other plugins to override this part, but by definining it as a part allows us to access the schema from anywhere without knowing the specific path to where it is located on disk. Another use case would be to provide the actual schema through a plugin instead of through the studio.
If you are encountering this error, it usually means that you have tried to implement a part that should not be overriden. If you think that the part in question is something you should be allowed to override, reach out to us.