As per the documentation, the free tier limits the name of the database to 63 bytes.
https://docs.atlas.mongodb.com/create-new-cluster/#atlas-free-tier
When I try this on Atlas free tier the error says Max database name length is 38 bytes.
Cluster0-shard-0:PRIMARY> use TEST_18c03cb3-8e88-48a3-904b-e2672659532e switched to db TEST_18c03cb3-8e88-48a3-904b-e2672659532e > db.coll.insert({a:1}) WriteResult({ "writeError" : { "code" : 8000, "errmsg" : "Database name TEST_18c03cb3-8e88-48a3-904b-e2672659532e is too long. Max database name length is 38 bytes." } }) >