Is it really your device?
Some unlicensed devices and even some batteries are not
compatible with CTS. However, there have been cases where Nexus owners bought
their device directly from Google and experienced the same problems. In that
case, return it to Google to solve it. An unlocked bootloader can also make
Google Pay quite difficult to use.
Solutions
Replace battery
This could work if you are using a really cheap battery.
Repositioning device
NFC antenna
It can be difficult to align the NFC antenna on certain
devices, such as Note 4. Try to hold it horizontally. This Samsung site
presents it by device.
Ask the cashier for help
This is a bidirectional process. If your device vibrates and
you get a green check mark, try asking the cashier to select Credit Card.
If you see this, ask your cashier to choose a credit card.
Use ADB to block the bootloader
Android Debug Bridge can be used to restart and replace the
original device manufacturer's lock on your device.
Lock the bootloader with ADB
Ask the operator / manufacturer to repair / replace the
defective device
There have been some cases, on devices such as Note 8, where
some units shipped from the factory do not have the secure element incorporated
in the NFC and security settings. You can verify this on your Samsung Note 8 by
going to Settings> Connections> NFC and payment> Touch and pay.
https://www.keyquery.com/internet/google-pay-not-working/