Kathi - based upon your attachment and the ArcGIS Pro Supported DBMS's (2.5, 2.6, 2.7), the only Oracle release lower than Oracle 12x is Oracle 11g R2 (64-bit) 11.2.0.4
In reviewing the details, Oracle 11g R2 (64-bit) 11.2.0.4 support goes all the way back to ArcGIS Pro 2.1
Anything other Oracle release is not supported and may be the culprit here.