The file is encrypted using a password. That password is apparently different than the one you are signed in with. It thus does not help to enter it again, it’s already tried. That’s why the message says you’re already signed in with that password.
I’m wondering – did you encrypt the document, or did someone send it to you?
If someone sent it to you, they need to use the key sharing feature in order for you to be able to open it. If they do this, the file is updated to include a key for you so you can open it – but they will need to resend the file to you after adding you as a recipient. All this is explained on our web page https://www.axcrypt.net/ .