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Express should work without a license; but "premium" things like symbolics, programming, and solve blocks will no longer calculate. A high level file will open using Express and show all the content, but if it's recalculated these will no longer display.
A license (more than a month) is
In addition to what Fred said, if you only need the simpler Premium features, such as mean(), Re(), Im() and more, then you may get by, for free, with Prime express and one or more of the functions of the sheet I presented here:https://community.ptc.com/t5/PTC-Mathcad/Prime-Express-A-joke/m-p/604216
Success!
Luc