Some logic to set up free access plans that require PMPro membership level to enroll in.

pmpro_lifter_llms_plan_get_checkout_url.php

/**
 * Require a specific membership level IDs to enroll in free access plans. 
 */
function my_pmpro_llms_access_plan_is_available_to_user( $access, $user_id, $access_plan ) {
	// Bail if streamline is not enabled.
	if ( ! get_option( 'pmpro_lifter_streamline' ) ) {
		return $access;
	}

	// If the plan isn't free, let LifterLMS handle it.
	if ( $access_plan->get( 'price' ) > 0 ) {
		return $access;
	}

	// Allow members of level IDs 1, 2, or 3 enroll in free access plans.
	$level_ids = array( 1, 2, 3 );

	if ( pmpro_hasMembershipLevel( $level_ids, $user_id ) ) {
		return true;
	}

	return false;
}
add_filter( 'llms_plan_is_available_to_user', 'my_pmpro_llms_access_plan_is_available_to_user', 10, 3 );

/**
 * Redirect members-only plans to the Membership Levels page.
 */
function my_pmpro_llms_plan_get_checkout_url( $url, $plan ) {
	// If the plan isn't free, let LifterLMS handle it.
	if ( $plan->get( 'price' ) > 0 ) {
		return $url;
	}

	// If the plan isn't available to the user, change URL to levels page.
	$available = $plan->is_available_to_user( get_current_user_id() );
	if ( ! $available ) {
		$url = pmpro_url( 'levels' );
	}
	return $url;
}
add_filter( 'llms_plan_get_checkout_url', 'my_pmpro_llms_plan_get_checkout_url', 10, 2 );