If you suddenly can't add products:
1. First check whether you've hit your plan's listing limit or have an unpaid upgrade invoice, since both can block uploads and the store itself.
2. If neither applies, send support a screenshot of the error and the store where it happens; than we can re-enable uploading. This usually takes some hours rather than being instant, and support confirms in the chat once you can add products again.
3. If the block was caused by a listing-limit or billing issue, resolving that (paying the correct invoice or upgrading the plan) is what restores uploads.