A family of Microsoft word processing software products for creating web, email, and print documents.
What is the nature of the differences between the documents? Different formatting? Different content? Both?
In particular, note that if differences were created as Tracked Changes, you may have to accept the changes in one version and reject them in the other version of the document, before you do the comparison.
Also, what is the result you are looking for? Perhaps a Combine is a better choice than a Compare? For more, have a look at https://support.microsoft.com/en-us/office/compare-document-differences-using-the-legal-blackline-option-dbfc7351-4022-43a2-a0c4-54d1898702a0.