Apple watch series 4 serial number not recognized

Check you have found the real serial number, and check more than one way of finding it. The number in the band amounting slot is particularly difficult to read, so easy to misread. Also be cautious about the potential confusions like 0 0r O (number zero or letter “O”). How to find the serial number or IMEI for your Apple Watch - Apple Support

What are you trying to do? Do you still have the packaging material that came with the watch? Does the serial number on the package match the one on the watch or in your Watch App on the iPhone?