According to the errmsg you have already run rs.initiate(). Your replicaset has already been initialized. You can check your current configuration using rs.config() in the mongoshell(i.e. mongos) or using db.getSiblingDB('local').system.replset.findOne()(as mentioned by @Stennie in the comments).
If you did not get { "ok" : 1 }, its because you did not give members in initiate arguments. Its no problem mongo took default configuration and created the current database into a replset primary.
If you were having a problem with pushing to database using prisma in local something like Prisma needs to perform transactions, which requires your MongoDB server to be run as a replica set. in mongosh local instance,it should be fixed now.
You can add members to your replicaset refer documentation.
Some resources I refered:
PS: I am also new to mongo. This is what I understood after my research.