Can't go from weak ref to strong ref in sketcher

[ptc-2646863]

This is the Student version of Pro/E, and I'm on p 83 of the tutorial. I am unable to dimension the lines on my sketch - I enter dimensions and press enter but they stay as weak dimensions.Feature Requirements dialog says "Missing References". What am I doing wrong?
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[ptc-1701983]

There is a config option that can be set to lock the dimensions once edited. The missing references sounds like the sketch is not placed properly or you have some extraneous references. Use the sketch references dialogue to check and ensure that the sketch is fully placed and that there are no references listed as missing or failed.

[ptc-1151073]

You can also right click on a dimension and set it to strong or locked. Casey