If you suddenly can't add products, 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.
If neither applies, send support a screenshot of the error and the store where it happens; they escalate a genuine "can't add products" block to the tech team, who re-enable uploading — this usually takes some hours rather than being instant, and support confirms in the chat once you can add products again.
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.