Is this an error? Erioed meant "ever" in previous sentences, not never which would be the opposite
The full form of the sentence is 'Nac ydw, dydw i erioed wedi bod 'na'
'dydw i erioed' = I have never (lit:- not ever)
In speech this is 'dw i erioed' but the position of 'erioed' in the sentence shows that this is a negative.
How does this work without the verb in the negative?
The verb is in the negative - dydw i erioed wedi... - 'I have never...' It is just that the dy- of dydw i is often dropped in informal speech.