、、。
Oh no, I showed up early on purpose but if I keep chatting I'll be late.
Literal
Oops, going-to-the-trouble-of time-before came [despite-のに], chatting-[if-てたら] late-become-will [casual-しちまう].
Frustrated casual muttering. おっと is an interjection for 'oops,' 'whoa' — a beat of self-interruption when realizing something. ダベる is a casual/slang verb for 'chat idly,' 'shoot the breeze' — origin unclear but common in youth/casual speech. ダベッてたら uses the casual contracted progressive with small っ for emphasis. しちまう is a casual contraction of してしまう ('end up doing'), specifically the masculine/rough male-speech form. So 遅刻しちまう = 'will end up being late.' The せっかく~のに frame ('despite going to the trouble of X') sets up the ironic frustration of having shown up early only to potentially miss the meeting anyway.