Some people may have several Amazon accounts.
In my household of 4 people, we have six Amazon accounts. Only one has Amazon Prime that we pay $99.99/year.
If you have several Amazon accounts, you should set up your Echo with the one that has Prime. I believe all Echoes are shipped with the member's account already setup. If that account isn't the one that has Prime service, then you'll need to change it. It used to be Echoes came with 1 month free of Prime service. After that, the Prime service ends unless you pay for it. I don't know if Amazon is still doing the same.