"This value must be a function" error

There is no need to apologise for being a new or inexperienced Mathcad user - not fully understanding something is par for the course (or "to be expected", if you're not in golfing analogies!).   Even experienced, long-time users make mistakes, sometimes because they've forgotten a rarely-used case, didn't notice that a feature has changed in the latest version (*), or that old classic of making a typo and failing to spot it because they see what they think they wrote as opposed to what's there.

My initial thought, which seems to be supported by your latest screenshot, was that you didn't have a multiplication or scaling operator between the Ws and the parentheses. OTOH, you had parentheses after every W, which usually indicates a function, hence my question about what W was. This also shows the importance of posting a worksheet for quicker and more targeted support.

You don't need the equals sign to find the solution, just to display the result.  However, it is generally regarded as better practice to separate a definition and its display, as evaluating (=) an expression can result in undesired side-effects in an evaluation.  An example of this was Mathcad Prime's previous behaviour of assigning the result of evaluating a range variable to the defined variable.   Many people used this as a way to convert range variables to vectors.  However, range variables cause enough confusion as they are without reliance on side-effects, so this "undocumented feature" was fixed in Mathcad Prime 10 ... which, referring to my first paragraph, caught a number of people out when worksheets stopped working as they expected.

Stuart

Stuart