[addendum Okay, for those who have responded to my related queries, it's getting through at last why I cannot assign the SYS schema to a workspace.]
I've inherited a scenario in which APEX is the only permitted interface (using SQL Workshop). My predecessors created just two schemas and two workspaces, RO and RW.
As the INTERNAL user, I thought I'd see whether I could create a third workspace and populate it with my own DBA-privileged schema. I raise a ORA-1031 instead. If this is covered in some documentation somewhere, my old eyes are completely missing it. TIA for guidance.