Units

attach the worksheet.

Prime believes you're calling a function.  Multiply a by diffF, see the effect

°F is indeed a function.

You need a scaling function to convert degrees of Fahrenheit to Celcius or Kelvin.

You should provide some argument to it. For example:

The second error:

I do not understand the intent here. α is not defined above. There is a predefined constant α in Mathcad, which is a fine structure constant, but I doubt you intended to use it. Anyway it is not a function.

Third error says that variable is not defined, which is also true. It is not defined. Also not sure what was the intent.

You are most probably missing a multiplication between alpha and the subsequent opening bracket.

Or did you define alpha as a function?

Success!
Luc