If you're being prompted to upgrade when trying to connect a new device, it's probably due to already having 3 devices or more connected to a Basic account.
Basic accounts have a 3 device limit, which is why when you try and connect an additional device there is a prompt to upgrade, so that you can connect more devices. You can check out more about the device limit from here.
If you have any old devices still showing as connected to your account that you no longer use, you can unlink these devices to free up a space to link a new one. You can check/unlink old devices with the steps outlined here.