If you want to advertise again and remove this error message, you must go to a CSS partner. All you have to do is choose a suitable partner who will suit your conditions and who will be good enough for your business. You can also find a list of CSS partners on the official Google site. After you choose a suitable partner, all you have to do is contact them and arrange the transition to their CSS program. As soon as the so-called switch under the CSS service takes place, this error message will be removed and you can continue advertising as before.
However, if you are not interested in any CSS program because their price list or conditions do not suit you, you can still advertise under Google CSS, which means that you will use Google for this advertisement. To use Google CSS, just write to Merchant support and they will handle it.