If adding the number "0" (zero) in front of your seven-digit passport number does not work, try adding the letter "P." International conventions sometimes use the first character position to tell the system what sort of document you are using.
If you have a card with a three-line OCR panel on the reverse, look at the first character before the document number. THAT is the character / letter to place in the first spot on the web site giving you a problem.
P = National Passport
I = National ID Card
For example, I have on occasion, used my US Passport CARD in this field instead of my US national passport booklet number. Using the "N" character has worked in some cases. I think some of the site directions need to be clarified as regards this point.
Also, whenever you see this "box" on a Spanish web site, check to see if it is a pull down. Many Spanish web sites have a pull down for you to select EITHER "NID" or "Passport. "NID" stands for National ID Document. Choose accordingly.
For most EU folks, this is the card they all carry, at all times. It is a national ID document, that doubles as a health insurance card for traveling abroad in the Schengen / EU zone, and for other uses from country to country.
Lastly, some web site designers may be going to one box, replacing the pull-down and NID / Passport choice for users. In this case, adding the "P" or "N" to the first character might just do the trick.
I hope this helps.