Hey, welcome to Ziggit! It would help a lot in finding a solution if you include the code that produced the error. Without looking at the code, the only cause that can be deduced from the error message is precisely what the error message says, a type mismatch between usize and u64.
On a side note; I recommend looking at this thread on allocators to see why it’s maybe not a good idea to be using std.heap.page_allocator directly.