[Expression.Error] cannot convert null value to Logical

Hi there, 

Perhaps you could wrap your DAX with another if statement to protect against nulls like this:

PV_LE = if (if [PV_real] = null then 0 else [PV_real])
>= (if [PV_orig] = null then 0 else [PV_orig])
then [PV_real]
else [PV_orig]

You could also find the max pretty neatly in Power Query using this M Code as a new Column: 
PV_LE = List.Max({ [PV_real], [PV_orig] }) - Returns the maximum item in the list or the optional default value if the list is empty

Let me know if this helps!
Numa