Redirect away from the checkout page if you don't have a required level with Paid Memberships Pro.

my_template_redirect_upgrade.php

/*
	Redirect away from the checkout page if you don't have a required level.
*/
function my_template_redirect_upgrade()
{
	global $pmpro_pages, $pmpro_level;
	
	if(empty($pmpro_pages))
		return;
		
	if(is_page($pmpro_pages['checkout']) && $pmpro_level == 2 && !pmpro_hasMembershipLevel(1))	//change ids checked here
	{
		wp_redirect(site_url('page-explaining-you-need-level-1-first'));		//change url here
		exit;
	}
}
add_action('template_redirect', 'my_template_redirect_upgrade');

Line 11 only worked for me once I changed the following

$pmpro_level == 2

to

$pmpro_level->id == 2

I figured this out when referring to https://gist.github.com/strangerstudios/5573829

Thank you for this Gist, nevertheless!

This recipe is included in the blog post on "Redirect Away from Checkout if User Doesn’t Meet Membership Requirements" at Paid Memberships Pro here: https://www.paidmembershipspro.com/redirect-away-from-checkout-if-user-doesnt-meet-membership-requirements/