Unfortunately ceil, as a lot of other functions, too, only work on scalars and not with units. So you have to get rid of the units and add them again later.
This behaviour makes sense. Lets look at a variable x:=50.5 in (= 1.283 m = 4.208 ft).
What should ceil(x) return? Mathcad stores the lenght in the default unit (maybe m) and the dimension. So ceil(x) could be anything like 51 in, 2 m or 5 ft.
You could write your own Ceil-function, but the desired unit would have to be a paremeter of that function:
