making my error message show a blank cell when printed

I am using two drop down boxes to determine a end value that is the weight for a work out. the first box is the category (upperbody, lowerbody, etc) that the determines which list is set in another cell in a drop down box. so if lower body is set in the first box, only my list of lower body movements show up. This works great thanks to some help I got recently on the forum.

The problem I am facing is that I expect some lifts like squat to plug into a formula and return a weight based on a percentage calculation. When I choose a lift that is not based on a percentage calc. It shows an error (rightfully so).

What I need is that cell to show up blank so my students can write the weight in that they used on those types of lifts.

What I tried: I tried conditional formatting so that formula =ISERROR(reference) turns text white. The problem is the text stays white even when I change the drop down box to a value that returns a number.

since within the formula there is always a chance for an error to be returned is there a way to set the format for only what is showing up and not every possible outcome.

Thank you for the reading and thank you for the help!