First things first, double check the NFC function of your phone is turned on.

If you have an iPhone it should be automatically on.

For Android phones, go into Settings → Connectivity → and then toggle the ‘NFC and Contactless Payments’ slide to the ‘ON’ position

(https://www.samsung.com/uk/support/mobile-devices/what-is-nfc-and-how-do-i-use-it/)

Read this blog if you’re still having trouble.