In case you see an error which says “Device Limit Exceeded”, this means you need to deauthorize an old device to proceed with using your subscription features.
To deauthorize a device, go to your app Settings > under Pro Options > click on Devices to see a list of devices authorized right now. Tap "Deauthorize" on the device you wish to remove.
*Please note that deauthorizing will delete any downloaded songs on the selected device for Pro subscription users.