Variable undefined error when using a ranged variable in equation

You are defining a function H which is dependent on an independent variable sigma.t (which at the time of definition has nothing to do with the range variable of the same name!).

You cannot evaluate (the equal sign after the definition) a function definition.

So delete the = at the end and you have a valid function definition.

There is also no need to define sigma.t in front of that definition.

Now for the range variable and changing default unit to psi not working. This may called a bug in Prime.

You can define the range and evaluate it in a second region where you can change the unit:

Generally it has proven a good idea over the years of working with Mathcad and Prime to separate definition and evaluation in extra regions. While inline evaluations often work OK and as expected, now and then strange effects may occur (especially when working with matrices). That's why I've gotten into the habit of not writing

but rather

An important thing to remember is that you should be aware of the difference between a range variable and a vector! This is a constant stumbling block when working with Mathcad.
Unlike a vector, a range variable is not a collection of values, even though in Prime the representation of vectors and ranges unfortunately looks completely the same. A range can be seen as kind of an implicit loop, not a series of values.

Ranges should be used for

• indexing vectors and matrices
• plotting
• a for-loop in a program

For anything else you should use vectors. Starting with Prime 11 a vector can easily be created using the new "vec" function.

This said, you sure can use your range as the abscissa values when you plot the graph of your function H (which is a simple linear function):

But don't use this range for creating a table of values. While this look good

you already run into troubles if you try to assign this "vector" (its not a vector, nor is it a range!) a variable:

So to create a table of values you should use vectors:

Here you see a severe limitation of Prime - we cannot change the units inside of a matrix from Pa to psi.

One way around this may be to divide the vector by the desired unit, creating dimensionless values and maybe adding a header line which names the units we see: