You are seeing that error because you are not supplying a properly formed expression to the function. The function expects an expression in the following format:
So something like 1+2 or 5.8*6. You are missing the second value in your expression. To get it to work you could Just add zero to your number so it won’t change the value. Something like the following:
More info here. However as @johnkiniston pointed out the CEIL, FLOOR, ROUND or TRUNC functions should also do what you need and probably in a more obvious way, so I’d go with one of those instead.