Without the specific error message, here is what could have happened. Because it happens to me all the time...
If you have the compare screen open, then you make a change to one of the profiles, you will get an error message when you try to propose the merge. The error message is because something has changed on one of the profiles. So, if you have the compare screen open in one tab, making changes in another tab, just remember to "refresh" the compare screen before you try to propose the merge.