Please check the server requirements in the help folder or [item page](/products/simple-pos-point-of-sale-made-easy) and make sure that your server meet the requirements, specially `MySQL` mode `only_full_group_by` should be disabled.

Lastly, please check the server error logs for error details and let us know so that we can suggest.

I wou I wonder how to install printer

You should follow the printer install instructions from manual