English question

Go ahead = 你可進行你想做的事
這裹代表沒人坐,請坐吧

例如
John: Miss Chan, may I go to the toilet please?
Miss Chan: Go ahead

即老師准許

[Go ahead] is a phrasal verb used to show permission for sb to start doing sth

So here it means [you may sit her]. The following translation is for your reference.

這裡有人坐嗎?

沒有, 請坐/ 你坐吧。