You can use this function to redirect all users that not have any membership to this page, so the users will be "locked" in this page until they buy access
add_action( 'template_redirect', 'my_membersip_restrict' );
function my_membersip_restrict() {
global $pmpro_pages, $post;
if ( is_user_logged_in() && !pmpro_hasMembershipLevel() ) {
if (isset($post->ID) && !in_array($post->ID, $pmpro_pages)) {
wp_redirect( pmpro_url() );
}
}
}
The function should be added to wp-content/themes/kleo-child/functions.php
Regarding the invalid key activation this is an odd issue, try to deactivate all plugins and enable it back... and enable first buddypress and try again and check again, an alternative to this is to disable email activation
Possible solutions here :
https://wordpress.org/support/topic/invalid-activation-key-error-in-buddypress-registration
https://wordpress.org/support/topic/invalid-activation-key-message
Cheers
R.