When adding the watch, the watch mobile number is added with the international country code of the watch sim card such as +34 in the Country and area box and add the rest of the watch mobile number without the +34 in the 2nd box.  The Country and area code is the code for the sim card and not your country and area code, unless they are the same

So when adding the watch mobile number, choose Spain as the country with the country prefix code +34 in the 1st box “Country and area” and in the 2nd box add the remainder of the watch mobile number starting with 590 etc.