Please refer to the Headphones manual.
Here are possible reasons why your phone wouldn't pair with your headphones:
*Your headphone Bluetooth is not turned on.
*Your headphone Bluetooth is not discoverable.
*The pass-code used is incorrect.
*The headphones are already connected to another device and needs to be reset.
On the headphones manual, there should be a pass-code and instructions to turn on Bluetooth and make it discoverable.
Please let us know what error message you are getting and provide a screenshot.
Also, are you able to discover your headphones when searching?
What is the make and model of your headphones?
These questions will assist narrow down the troubleshooting to resolve the issue.
In most cases, there is a button on the headphones that you need to press and hold to activate Bluetooth and make it discoverable.