Although it doesn't state compatibility with ZC1.5.6 I have been running it in my upgrade to 1.5.6a since 156a was released.

environment Dev: Zen Cart 156c; Apache 2.4.33; PHP 7.3.1; MySQL 5.7.21; Windows 10
environment Test: Zen Cart 156a; Apache 2.4.29; PHP 7.2.4; MySQL 5.7.24; Linux 4.15.0

packages / add-ons:
ckeditor ; clone template 1.2.0; payment module fee; Direct bank Deposit V1.5.revised; zencart155 securepayxml (modified); ozpost v4.2.7;
ceon_back_in_stock_notifications.9 (modified); export_shipping_information_V1.3.5 (modified);
edit_orders-4.3.5; master password 4; admin log in as customer; ZCA bootstrap template-for-156-v2.0.0c; image checker 2.0; sales report 3.2.2; email archive manager 1-8e; zenNoncaptcha v0.5