Can someone please check if they see the same issue in SW 2018.
Set the document units to inches and try to add a global variable in mm or cm units
When SW can't make the most basic functions work 5 years after the first release.
How errors like that go past many years of quality control beats me. This error is in 2016 & 2018.
If your document length unit is set to inches then you cannot specify metric units in the equation manager.
But you can specify imperial units for GV if the document length is set to mm (metric).
SW 2018 adding a 12cm global variable with document units set to inches.
Side Note: Maybe one day SW can make this Equation Userform user friendly. It is such a poor design.