I think we use bukan to negate nouns. Bukan apel, tapi jeruk. Bukan satu, tapi lima. Bukan dia.
We often hear "tidak masalah", which means "no problem". I think it should be "tidak ada masalah", there is no problem, or "bukan masalah", it is not a problem, or "jangan dijadikan masalah", don't turn it into a problem.
People say "Itu bukan masalah saya", which means "That is not my problem". People don't say "Itu tidak masalah saya".