There are at least two problems in your sheet.
The most prominent one is a simple typo you can quickly find using "Trace error" as Alan had shown:
The system still will not solve after you delete that extra n and recalculate the sheet with ctrl-F9.
The error message as of the undefined variable is the only error message a solve block will throw, no matter what the real error is. Thas quite silly and irritating. If you now trace the error, Mathcad is telling you that it does not find a solution and suggests changing tolerance settings or changing the guess different guess values.
Now Alan's an my solution differ. Alan has deleted the front factor 4 in the equation above the one with the typo (why?) and the system solved.
I changed the guess value of n.T from 5 to 10 and Mathcad solved the system. Changing the guess n.T to 1 also does the trick.
Your system seems to be quite sensible as to the guess values.
Its also interesting to look which algorithms are chosen to get a solution. You can see and change the algorithm used when you right click on "Find". Three algorithms for nonlinear systems are offered and if "Autoselect" is checked, Mathcad tries its best to chose a suitable one.
In Alan's sheet only Levenberg-Marquardt does the job and the other two will fail.
With T.n=1 only Quasi Newton works, the other will fail
With T.n=10 Levenberg-Marquardt fails, the other two work.