This error occurs majorly due to three reasons. Please check the following configuration on your end.

1) Check if you have installed the latest version of Single Sign On extension in your WordPress site. ( Version 1.3.2 )

2) Check if wdmwpmoodle plugin is installed on your Moodle site. Also, check if you have installed the latest version of it. ( Version 1.3.2 )

3) Check if you have added wdm_sso_verify_token function to the External Services.

Please try verifying the token key again once these changes are made.

Let us know if this does not fix the issue on your end.