In Google Sheet, you can create 1,000 links in 24 hours. You can solve this problem by creating a new sheet or build a new page.

After creating links, please click "Update Clicks & Protect Short Links" in exchange of formula into plain text. In case of re-calling the formula after you open a Google Sheet.

Can "Not a Valid URL" turn back to the original links?

No, it can't because the links you create are also different from the original one. So please remember to click "Update Clicks & Protect Short Links" after shortening the link.