Can you tell me which LEGO set number this is? I bought this Lego set off the internet and I can’t figure out the set number.  Can someone tell me the set number?
As @mindstormsboi pointed out, it's a modification of 60009-1 Helicopter Arrest
According to BrickLink, the only sets with both those RBrown stairs and the 100 tile are:

10218-1 Pet Shop
10232-1 Palace Cinema
10246-1 Detective's Office
10255-1 Assembly Square
10264-1 Corner Garage
41108-1 Heartlake Food Market
60009-1 Helicopter Arrest
60069-1 Swamp Police Station
60130-1 Prison Island

and that doesn't look exactly like any of them. But if you remove 60009's  vehicles and pier and add a few bits, it's a good match.
I won't say never, but Lego doesn't often (that I know of) sell sets with treasure chests where the lid doesn't match the base. But @Zhaph - Ben Duguid noted that 60009 does.