Hi AmazonX,
Welcome to Apple Support Communities! We'd love to help in any way we can with your iPhone and speaker, or will be sure to point you in the right direction.
To start, let us know if you've had an opportunity to restart both devices before attempting again to pair them.
Also, what's the version of iOS installed on your iPhone 12? This can be found under Settings > General > About.
If it's not updated to iOS 14.6, we can recommend backing up your data, then updating. The following guide has steps to help:
Update your iPhone, iPad, or iPod touch
We'd also like to share this article with troubleshooting steps when pairing a Bluetooth device:
If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch - Apple Support
Is your iPhone able to pair with any other Bluetooth devices, as a test? We want to be sure to mention that if you should continue to experience issues and it's isolated to your JBL speaker, it may be necessary contact JBL directly at that point.
Have a great day!