There are a few different ways to fix the issue of the Apple Watch keyboard notification popping up:
1. Disable the "Scribble" feature: Go to the "Watch" app on your iPhone, select "My Watch," tap "General," and turn off the "Enable Scribble" option.
2. Disable "Quick Replies": Go to the "Watch" app on your iPhone, select "My Watch," tap "Messages," and turn off the "Quick Replies" option.
3. Disable "Handwriting Recognition": Go to the "Watch" app on your iPhone, select "My Watch," tap "General," and turn off the "Handwriting Recognition" option.
4. Disable "Siri": Go to the "Watch" app on your iPhone, select "My Watch," tap "Siri," and turn off the "Listen for "Hey Siri"" option.
If all above methods don't work, you can try to reset your Apple Watch. To reset your Apple Watch, press and hold the Digital Crown and the side button simultaneously until the Apple logo appears, then release the buttons.
If the problem persists after trying the above solutions, it is best to contact Apple support for further assistance.