there was a similar discussion 3 years ago in the community: 12.2 - Ora-03137 [kpoal8Check-5] from SQL Plus 8.0
TL;DR: Connections from Oracle 8 clients to Oracle 12.2 and higher are not supported
D2k Client Version 8i
Oracle8i is release 8.1. D2K (Developer 2000) is based on 8.0.
Why using 20 year old software? And expecting it would still work with 18c (...?!)
I assume the answer is: Forms 6i is the last client-server release of Oracle Forms
As far as I remember, Forms/Dev 6i can connect to 12c Release 1, but not to 12c Release 2 and higher.
And I found a MOS note on this issue "ORA-3137[kpoal8Check-5] from Incompatible Client Version Connecting to a 12.2 Database (Doc ID 2296523.1)"
can't argue or ask them to move upgrade to customer if customer are very happy with d2k platform, we are here to provide the solution.
If the customer is happy with an unsupported client tool, he should accept running an unsupported version of the database, too.
D2K runs fine with database versions up to 12.1
Just my 2 cent,