Hi Amar,
Regarding your issue, please find the required information specified below.
Can you please look for the db6level executable in the /sapmnt/<SID>/exe directory and execute that one to try if the db6level runs successfully.
And also check if the sapinst executes a db6level loaded on installation directory */CENTRAL/AS/DB6LEVEL/db6level is different from the ones loaded into exe directory.
If they are different please copy the db6level along with the requested libraries from the EXE directory to the installation directory and repeat the phase.
Thank You.
Regards,
Deepika