Please note that "If this seat is free" is not only acceptable but usual in UK English. "Available" is rather formal, and certainly not the only translation. Thanks.
And it is accepted when you type your own answer.
If the seat is free, I'll sit down. Not correct?
Duo normally rejects switching tenses from futur proche to futur simple (and vice versa).
The French sentence uses 'ce' so it is important that you replace the 'the' with 'this'. I entered "if this seat is free, i will sit down." It was accepted.
Why not, "if this seat is open"?