A new user gets E2028 USER DOES NOT EXIST, TRY AGAIN when trying to use DMLO.
IDMS - all supported releases
One possibility for the E2028 error is the DMLO userid is not a valid userid. Another possibility is that the DMLO Userid is not defined in IDD. If that's the case you need to add this DMLO userid to IDD for the dictionary that accessing DMLO. DMLO does not use central security and thus an IDMS Userid and DMLO Userid are not identical.
DMLO uses IDD security instead of central security, therefore the DMLO security program DBMSDMLO requires a USER-047 record in the IDMSNWKA subschema to validate dictionary security via IDD.