VBA Macro Not Working When Workbook Is Protected

I wish I could have been more specific with my Title. This is what I am encountering. I used the McGimpsey code for generating a unique sequential number every time a workbook opens. I tailored the code to fit my needs and stuck it into my invoice sheet. For the longest time I was unable to make it work until I moved the code from a template to a standard workbook. Then I was able to make it work as long as the sheet was unprotected. I realized that to get the code to work I had to then unprotect the cells that the code was writing to. Now that I've done that I am getting errors in the code that weren't there before, rendering the macro useless. The code I am using is: