Set Parameter in SPFILE still outputs the same error
Digging further: Your error indicates that you are using a container/pluggable database (CDB/PDB) architecture for your Oracle Database. The SAP IDM installation guide specifically says not to do this:

When installing the Identity Management database on Oracle, note the following prerequisites and recommendations:...
...make sure the Create as Container database option is not selected

https://help.sap.com/viewer/d77277f42c0b469db8794645abd954ea/8.0/en-US/571119cc37ce465b93de726fc0408427.html
Based on this, you will most likely need to recreate your database from scratch and make sure it is configured as a traditional instance, and not as container/pluggable database architecture.