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This is a known bug:
Considering that Prime 3.1 is very old software (Prime is at version 7)
And this bug was reported more than a year ago
I doubt if it will be fixed.
But maybe this helps:
https://community.ptc.com/t5/PTC-Mathcad/Getting-Started-Tutorial-not-working/m-p/254149
Success!
Luc