Ran into this issue during a fresh install of Oracle 18c. Installed ORDS into the PDB not CDB.
Tomcat 8 as application server.
Started seeing non-stop errors in the alert.log
PDB(3):WARNING: too many parse errors, count=561200 SQL hash=0xcd9b76f4
PDB(3):PARSE ERROR: ospid=29325, error=904 for statement:
PDB(3):select dbms_service_prvt.get_topology('pdb') from dual
PDB(3):...Application: Oracle REST Data Services Action:
Fixed it by executing "grant execute on dbms_service_prvt to ords_public_user"