Error provisioning namespace. ORA-20001 Request could not be processed at Oracle Apex

I finally managed to install Oracle Apex 5.1.2 but I have problem with creating a workspace. Whenever I try to do so at the end I get an error: I tried to create this workspace with following values: The strange thing is that when I try to use Yes as option to Reuse Existing Schema no schemas are listed. Is it possible that Apex somehow doesn't have access to managing schemas?

I am using APEX with ORDS. At home page I get info that I have 1 workspace and 1 schema.

I've tried:

• Using strong passwords as mentioned here
• Changing provisioning type to request: Effect is the same. If user request a space and I accept it I get the exact same error.
• Enabled OMF with parameter DB_CREATE_FILE_DEST = '/u01/app/oracle/oradata' -> *.dbf files are not created before and after the change in directory.

The root cause of this problem was installing APEX both on CDB$ROOT, so as a result, and on PDB1. I uninstalled APEX from root, repaired with @utlrp.sql script as in this tutorial and installed APEX again, but only on PDB1. Workspace was successfully created.