Require a certain level before registering for another level with Paid Memberships Pro.

my_pmpro_registration_checks.php

<?php   // do not copy this line.
/**
 * This recipe Requires a certain level before registering for another level.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 * Edit lines 19,20,33,34 with your site levels. Update message line 22
 */

function my_pmpro_checkout_before_form()
{
    global $pmpro_level, $current_user;

    // Check if the user is trying to checkout for the specific levels and doesn't have the required level
    if(
        ($pmpro_level->id == 2 && !pmpro_hasMembershipLevel(1, $current_user->ID)) || 
        ($pmpro_level->id == 3 && !pmpro_hasMembershipLevel(8, $current_user->ID))
    ) {
        pmpro_setMessage("You must fill out your Student member profile  before registering for this course.", "pmpro_error");
    }
}
add_action("pmpro_checkout_before_form", "my_pmpro_checkout_before_form");

function my_pmpro_registration_checks($okay)
{
    global $pmpro_level, $current_user;

    // Check if the user is trying to checkout for the specific levels and doesn't have the required level
    if(
        ($pmpro_level->id == 2 && !pmpro_hasMembershipLevel(1, $current_user->ID)) || 
        ($pmpro_level->id == 3 && !pmpro_hasMembershipLevel(8, $current_user->ID))
    ) {
        $okay = false;
    }

    return $okay;
}
add_action("pmpro_registration_checks", "my_pmpro_registration_checks");