APEX ADD EXISTING SCHEMA TO WORKSPACE throws ORA-1031

Oracle EE 19.0; APEX 1901; Amazon Web Service (RDS)

I recently made my first attempt to add a workspace for our DBAs, and to authorize the schemas for it. As user APEX_190100 I did the prework to unrestrict the DBA schemas. Then from the APEX app, I connected as the admin: | manage workspaces | manage workspaces and users | add (existing) schema to workspace. Insufficient privileges, every single time.

I have not yet found any command-line syntax, either using CLIcl or RDSADMIN. Any insights are appreciated.