Shopify's checkout doesnt recognize Postal Codes - "Your order cannot be shipped"

[Witko]

Hi. I have a problem with my checkout. I do offer shipping to some countries in the EU and US, but when you add an item to your cart and go to checkout, enter your address, message "Please check your address to make sure it is correct and try again or choose a different address." appears. I checked a few things and I think the problem is in the postal code.

I live in Poland, so I just entered my address (postal code looks like xx-xxx) and yes, I offer shipping to Poland, I also tried in different countries and this message appeared every time at every address.

For the shipping and delivery settings, i just added them myself (named "free shipping") because for the "Use carrier or app to calculate rates" i don't get any options.

I use Zendrop to import products, but I don't think that is causing this problem. Does anyone know how to fix this?