Hello JovanaNovoselski,
Welcome to Apple Support Communities. We understand wanting to trade-in your iPhone, and you're getting a message about not having enough credit on your Apple Card. We definitely want you to be able to upgrade to iPhone 12, and we'd like to help.
To get started, please try the following link:
iPhone 12 Pro. It’s a leap year.
Select the model you would like, and follow the prompts. Then select the option to trade in, and enter the information for your current iPhone. You'll get an estimate for your trade-in, and the option to pay in full or make installments using Apple Card.
If you still having problems completing the trade-in, please contact the Sales Support team here:
Contacting Apple
We hope this information is useful. Kind regards.