……、、。
Ugh... I was young, sure, but what a stupid rookie mistake.
Literal
Ahh...., was-young [even-though-とはいえ], pointless common-mistake [copula-だ].
~とはいえ 'although / even though / granted X' is a formal-ish concessive conjunction, heavier than ~けれども and with a slight written tinge. しょうもない is a colloquial 'pointless / silly / good-for-nothing' derived from 仕様もない '(having) no way (to help it).' 凡ミス is a compound 凡 'ordinary / commonplace' + ミス 'mistake' = 'a rookie / careless mistake.' The concessive sets up 'granted, I was young,' then the evaluative follow-up lands.