Dear lorant124,
Thank you very much for your feedback.
We are very sorry that you are experiencing difficulties. However, we find it very difficult to help people like yourself.
Giving a one-star review without contacting our support team is not very nice and hurts the whole team. We spent hundreds of hours developing this plugin and deserve as much as a hint about a problem before getting a slap. Do not forget that even though it is not an excuse, the piece of software you are holding is FREE.
The current version of ATUM runs smoothly on all the latest systems (WP 4.8, WC 3.1.0).
We hope you can give us little more info about the error above and consider correcting the starts in your rating.
Hope to hear from you soon.
Pavel