Is it only possible to get Apple Card if you are a US citizen?

No. See the following from How to apply for Apple Card - Apple Support

• Be a U.S. citizen or a lawful U.S. resident with a valid, physical U.S. address that is not a P.O. Box. You can also use a military address.

At this time you must be at least 18 years old and a US citizen or lawful resident. See here: https://support.apple.com/en-us/HT209257