Hello @NoemiJ,
sorry, no solution to your question, but maybe this helps a little further anyway:
It seems to me, the backup/import only work, if you have multiple mobile devices, logged in with the same profile and activated timeline with backup on each or you need to switch to a new device, as the description suggests “Save an encrypted copy of your Timeline in your account, in case you need to switch devices.”
In this case, the menu does allow an import, but only from the other device, not from the same:
What I don’t know, whether there is a way to start a backup now. The description says “Timeline backs up to Google servers over Wi-Fi when your device is charging and idle.” The question is, when is the device considered “idle”? Even, if I used the other device yesterday, the backup is always several days old. Besides that, it says in the more detailed description, the whole day is ignored, if both devices were online at the same place.