Unable to locate bitcoin.conf in linux
You showed a directory listig using "ls". Maybe that is the first issue. Try using "ls -la", which would also show hidden folders, beginning with a dot.

Otherwise here is a tutorial:
https://bitcoin.org/en/developer-examples#regtest-mode
and
https://bitcoin.org/en/full-node#linux-instructions

difficult to say, what happened, but if you just create a ".bitcoin" directory, and go ahead to create the bitcoin.conf file, you should be ok.