You are applying a pressure, so your expression needs to evaluate to either a unitless value (in which case the system assumes pressure units) or to pressure units. The only variable in the expression that has units is 'y', which has length units, so 4.1233*(y-50) evaluates to length units.
To fix this, divide the y value by 1 length unit. If you are working in mm , the expression would be:
You forgot the key part - the units designation on the 1.
ug_var("y") returns a length unit. Dividing it by a unitless 1 still leaves a length unit. You need to divide by 1[mm] to have the units cancel:
the expression should be entered exactly as follows: