I already defined the solve block variables but mathcad is still giving me an error that shows that the variable/equations are undefined.
Can anyone help me find the error here so I can find variable P?
You first need to fix a units problem. Mathcad insists P has units of force, in which case sigma(P) has units of pressure. It therefore cannot be equated to sigma[n, which has units of force.
As Richard said, it's a units problem.
Your solution would be:
But when evaluated: