Thanks for reaching out to the Roku Community.
We will go through these details with you. Can you let us know if you obtained your Roku device serial number from the device itself or from the Roku device setting menu? We recommend going to Settings > System > About, as you can find the device's serial number and associated Roku account email address on this page.
After verifying the email address associated with your Roku device, please try logging into your Roku account again. Once logged in, scroll down to the "My linked devices" section and locate your device to verify that it is listed there.
Keep us posted on what you find out.