Compare and merge error message when trying to compare and merge two different workbook

[SH2020]

Hi guys. Can someone help me with this Excel problem? I have a problem with my excel file that made it really hard for me to do my homework. I'm supposed to use the Compare and merge button and compare and merge my second workboot into my first workbook, but when I do that I get an error message. Also, most of the tabs in the Ribbon are grayed out like the comments (previous and next comment) and when I click the show comments tab, I get the "there are no comments in this file) message even though the file has comments. Here are pictures.