What were you trying to do that didn't work?

leapp reports correctly that Berkeley DB (libdb) has been detected on your system. A Berkeley DB format database is /etc/aliases.db. You cannot change postfix to use another DB format AFAIK.

The other Berkeley DB files I have found are /etc/openldap/certs/*.db:
cert8.db key3.db secmod.db

Trying to use db_converter on the files in question fails with errors like:

BDB0210 secmod.db: metadata page checksum error
Can't open database - 22

As per customer's request: 

 
1.
Is there a replacement for Berkeley DB in the RHEL 10.0 version of postfix?
2.
Is there a replacement for Berkeley DB in the RHEL 10.0 version of openlap?

You cannot simply use another DB with the existing versions of these two packages.

Please provide the package NVR for which the bug is seen:

libdb