This is happening because a separate nonce with the action wp_rest is not being sent by the server to the client and received back from the client in an HTTP request header called X-WP-Nonce with every REST request.
To get this working, you will have to generate a nonce like this:
wp_create_nonce('wp_rest')
…and provide it to the client making the rest call. Once your client has the nonce value, you need to add it to every REST request e.g.:
headers: {
'X-WP-Nonce': nonce,
}
Creating the nonce on the server and accessing it on the client can be done several ways. Using wp_localize_script()
is the most common and probably best practice for WordPress. wp_localize_script()
addds a global variable to the client for a script to access. See https://developer.wordpress.org/reference/functions/wp_localize_script/.