When I click convert, I receive a message that reads "an error occurred while signing in" however I am signed into my account already
As per the description given above, I assume that you are trying the conversion online via- https://cloud.acrobat.com/exportpdf ?
You may try with a different web browser and make sure that after signing in you are able to see your name at the upper right corner.
You can also use the services through free desktop version of Adobe Reader- Adobe Acrobat Reader DC Install for all versions
You just need to “Sign In” using the email (Adobe ID) under which you have subscription registered.
Try this, and let us know it if it still doesn't work.