I had the same problem with Word 365 after installing Mendeley update. Here is the solution I found out, it may help other researchers.

Step 1: Open Word, then go to “file”->“options”, a new window pops out.

Step 2: Select “Add-ins” in the left, then select “word add-ins” in “manage” (at the bottom), and click on “Go”

Step 3: Click “add”, then select “Mendeley 1.19.4.dotm” from the Mendeley installation file, it may be in “C:\Program Files (x86)\Mendeley Desktop\wordPlugin”

Step 4: Click OK, then you should find mendeley in the “reference” tag on the top of word 365.

Original: https://www.researchgate.net/post/How_to_add_Mendeley_plugin_into_word