Maybe you have already solved it but I leave my solution in case someone else finds it useful:
I had that problem and in my case it was because I could not filter a field that has the attribute “private”.
my query looks like this: api/products?filters[category][id][$eq]=${myCategoryId}
you can see it in the schema.json: api/product/content-types/product/schema.json
or you can also check in admin panel, in your case products > types > advanced settings and check that the private field checkbox is unchecked.