VBA Message Box Worksheet Change

first off, your link is to a shared FOLDER and I just guessed the file name as "VBA Question 2" because it was new. Not sure if you care to share all those files but you should either share the file or tell us which file to look at 🙂
That said, I first offer an easier and possibly better solution. In that file I created a simple IF statement to do what you want (cell G20) added with some conditional formatting to make it stand out and very clear notification. The reason I recommend this is that if they open the file online macros don't work, they also don't work if they have security settings at high or medium and decline letting them run. So this solution work in all cases and you can even lock the sheet (obviously unlock the cells they need to use/enter info into, which has to be done in the desktop version but works online) to prevent them from accidentally deleting that cell or hiding that row.