Every time I open the Wallet the plus sign does not appear

Go to settings>general>language and region and select 'United States'. Then restart your device and you show now see the + sign to add cards in Wallet. Be aware that, in order to apply for the Apple Card, you must have legal authorization to live and work in the US (i.e, be a US Citizen or lawful permanent resident) and have a credit history there.

Go here to make sure your phone is set up correctly - Steps to take for applying to Apple Card - Apple Community