Error: Inconsistent Units in Revit

Issue:
When creating a formula to calculate a parameter in a family, the following error appears:

Error:  Inconsistent Units

Solution:
This message appears if the units are inconsistent. For more explanation and a solution, see the links below:

https://knowledge.autodesk.com/support/revit-products/troubleshooting/caas/sfdcarticles/sfdcarticles/Inconsistent-units-when-creating-formula-for-calculation-in-a-family-in-Revit.html
https://www.revitforum.org/tutorials-tips-tricks/1138-revit-inconsistent-units-how-neutralize-them.html