I have been able to replicate this error by starting up MongoDB 4.4.15. Once it was running I stopped it. I then started up MongoDB 6.0.0 and pointed it at the same path that the 4.4.15 version wrote its database files to and I got the FCV error and the process stopped. This seems to be what you are seeing.
If you want to save your data, you can fix this, by downloading the compressed archive of 5.0.x and extract it. From this extracted folder you can run ./bin/mongod --dbpath <current database path>. Connect to this instance with mongosh and run db.adminCommand( { setFeatureCompatibilityVersion: "5.0" } ). This will change the FCV for you. You can then exit mongosh and then shutdown the mongod instance and finally start your version 6.0.0 server. You will want to change the FCV here as well to be 6.0.
If you don’t care about any of your data, you can just delete the entire database directory that the current data is stored in.