If you receive the error message "You must enter a valid UID number", first verify that you entered your VAT ID without the country code. For example, if your VAT ID is DE123456789, enter only 123456789 when selecting Germany as the country.
If the number is correct, the VAT Information Exchange System (VIES) may be temporarily unavailable.
Another possibility is that your VAT ID is not approved for cross-border business transactions. Since purchases are processed by Paddle, which is based in the United Kingdom, your VAT ID must be valid for international VAT verification.
You can check your VAT ID here: https://ec.europa.eu/taxation_customs/vies/vatRequest.html
Keywords: UID, VAT number, VAT identification number, VAT