We would recommend performing a factory reset on the affected device and then trying linking your existing Roku account once again.
Performing a factory reset using the hardware reset button
On the back or bottom of each Roku device is a reset button. On some devices, there is a tactile button, while other devices have a pinhole button.
If your device has a pinhole button, you will need a paper clip to perform the factory reset.
Once the reset button is located and your Roku device is powered on, press and hold the reset button firmly for about 10 seconds. The indicator light or status LED will blink rapidly on most Roku devices when the factory reset is complete.