Themes

I believe this message comes from the following code (the first if block).
It looks like the call to can_create_contact() fails for some reason. Maybe your user or role has no capability of some sort?

File: message/externallib.php

if (!\core_message\api::can_create_contact($params['userid'], $params['requesteduserid'])) {
    $result['warnings'][] = [
        'item' => 'user',
        'itemid' => $params['requesteduserid'],
        'warningcode' => 'cannotcreatecontactrequest',
        'message' => 'You are unable to create a contact request for this user'
    ];
} else {
    if ($requests = \core_message\api::get_contact_requests_between_users($params['userid'], $params['requesteduserid'])) {
        // There should only ever be one but just in case there are multiple then we can return the first.
        $result['request'] = array_shift($requests);
    } else {
        $result['request'] = \core_message\api::create_contact_request($params['userid'], $params['requesteduserid']);
    }
}

Edit:

And indeed it very much may be a problem with configuration. Could you please check if you have messaging enabled? And if both users are sharing the same course?

File: message/classes/api.php

/**
 * Checks if a user can create a contact request.
 *
 * @param int $userid The id of the user who is creating the contact request
 * @param int $requesteduserid The id of the user being requested
 * @return bool
 */
public static function can_create_contact(int $userid, int $requesteduserid) : bool {
    global $CFG;

    // If we can't message at all, then we can't create a contact.
    if (empty($CFG->messaging)) {
        return false;
    }

    // If we can message anyone on the site then we can create a contact.
    if ($CFG->messagingallusers) {
        return true;
    }

    // We need to check if they are in the same course.
    return enrol_sharing_course($userid, $requesteduserid);
}