How to grant SYSDBA access to users on Oracle Cloud v19c
In short, you can't. You need to use PDBADMIN user or PDB_DBA role for such things: https://docs.oracle.com/en-us/iaas/exadata/doc/eccmanagingdatabases.html
Also there is a very useful document:
Known Issues for Oracle Database Exadata Express Cloud Service
It gives some more internal info about cloud access and privileges.