Bolla is incorrect. Maybe you've heard wrong.

몰라(mol-la) is correct.

Sorry Im not a native korean speaker but I think I can help you. (Molla) means "I dont know" (Bolla) is how some people will pronounce it. I would say Molla is the best pronunciation since its spelled with a M not a B (몰라). Since Im not always correct, I hope a native speaker can answer this for you and give a better explanation or helpful tips. :)

i just say bo-lah-yo i think it means the same thing