！
If you've got time to mess around on acoustic guitar, go practice the bass!
Literal
Acoustic-guitar [or such-なんか] strumming [and-て] playing-around time [subject-が] exists-[if-たら] bass [possessive-の] practice do-[imperative-しろ]!
アコギ is the colloquial shortening of アコースティックギター (acoustic guitar). なんか trailing a noun is a dismissive 'stuff like X', belittling the activity. The frame [verb]-ている暇があったら、[verb]-しろ ('if you have time to X, do Y instead') is a productive scolding pattern. しろ is the plain imperative of する, the abrupt form used for orders — rarely heard outside of scolding, sports, and masculine speech.