Error: This value must be a scalar or a matrix

[CN_13593194]

I checked other posts about this issue, and I couldn't find one similar to what I am doing here

I am wondering if that is because of the sin function or what

mathcad 9.0.0 file is also attached 

[Werner_E · Accepted solution]

The question was also answered here 

You have to manually label the two deltas as "Variable" or "-" = "Automatic"

[CN_13593194]

would you happen to know why only delta was defined as 'function'? but not the other ones (i and Mu)?

I am wondering if I copied and pasted it from somewhere else or if it's because it is a Greek letter

[LucMeekes]

It is due to the intricacies of the enigmatic 'autolabelling' feature of Prime.

It's a nice feature, and often it helps. Too often it strikes wrong.

What helps is if you make all label types look differently, e.g.:

Then you immeditely see.

Success!

Luc

[Werner_E]

Primes auto-labelling feature fails quite often. 

It often fails when used in combination with symbolic evaluation and also when you start editing an expression and using copy & paste.

So I guess it had nothing to do with the Greek letter but was because of editing and using copy and paste.
Lucs suggestion sure will help spotting that kind of error.