“All contacts” are contacts you created and Other Contacts you’ve emailed.
Select Google CSV format.
In the top right, click your photo, then click Sign out.
There are several reasons that may interfere with importing your contacts. The most common are the incorrect file extension. That is the first place to check then you should check to make sure that your formatting is in place.
Keep in mind that the headers should read First Name, Last Name and Phone Number.
Make sure that you have checked the box to Opt In your contacts or you will not be able to send messages them.
If you have a large list of contacts there could be delays which may seem like the operation is stalled but give it a few minutes, it is more than likely just working on importing your data into the group. Just be patient.