"I was not there."
Translation:Cha robh mi an-sin.
The word ann on its own tends to mean 'exists' so sometimes in some contexts phrases using it translate into English as 'there is', e.g. Tha aran ann - there is bread but more literally 'is bread in existence' However if you want to use the 'there' in english that means a place to distinguish it from here, you use an-sin. Here an-seo, there an-sin. So while it is possible that cha robh mi ann might be translated to I was not there, it would have overtones of your non-existence. For phrases having to do with location rather than existential uncertainty stick to an-sin
In the case of a person that may translate to "i did not exist". But , i am not sure. "Ann" on it's own more directly translates to "in existance" or "exists". I cant really say though, we need more than just the definition to know if the context would alter the meaning. Hopefuly a kind person could clarify.
Cha robh mi ann: I wasn't there (in that place/at that gathering(large or small). I would not relate it to existence, which would be 'cha d' rugadh mi aig an àm sin': 'I wasn't born at that time'. Cha robh mi an sin: I was not there, as referring to a specific location/occasion.