Find the mistake:
It's essential for us to book tickets for the theatre in advance .
I see no mistake here, can anybody help me find the mistake?
Find the mistake:
It's essential for us to book tickets for the theatre in advance .
I see no mistake here, can anybody help me find the mistake?
It's entirely grammatical. Those two fors in different senses (one as a subordinator, the other as a preposition) excite my horror aequi, and I'd rewrite as "It's essential for us to reserve theatre tickets in advance" or "It's essential that we reserve tickets for the theatre in advance" or even "It's essential that we reserve theatre tickets in advance"—but this is a stylistic matter, not an error.
The consensus of our experts as read in the earlier answer and the above comments seems very much to be that there is no mistake here.