"Backup Now" is greyed out

I tried to use Deja Dup for the first time on a computer. I set the backup destination, an external hard drive, and scheduled a weekly backup. However, I get a series of windows asking for my password. When I give it, the window says that a backup is starting and there is some activity there. However, a few seconds later I get another window asking for a password. This goes on and on and on ... Then I opened the settings window for Deja Dup but the "Backup Now" button is greyed out. I do not know what is wrong. Deja Dup works on my other computer. Does anyone have any ideas?

I am using Ubuntu 16.04 on an old Sony notebook. It has 64 bit words.