Poland is supported, but this means you need to have a Poland passport that you have provided to Uphold. If you have not done this, then you can’t connect to Uphold at this time.
If you are using from Ukraine, then Uphold would report to Brave that you’re from Ukraine even if you’re living in Poland and provided Poland residence. So this is the very first thing you need to resolve.
Step 2: Operating System region:
If you are using Russia for your Country, Regional Format, and sometimes as language then the browser will see you from Russia. Therefore you need to change that.
Step 3: Your Rewards country:
When you first use Brave and set up Rewards, it asks you to choose your country. If you chose Ukraine but use Poland passport, then it would give you an error that says Country Mismatch. You would have to Reset your Rewards and choose Poland for your county. You can reset Rewards by going to Rewards in your settings and then choosing to Reset. Of course, if you have any vBAT on the device it will be lost when you reset.
You would need to contact Uphold. You can do a Uphold Support Ticket at https://support.uphold.com/hc/en-us/requests/new where you would ask them to update your information from Ukraine to Poland if you have not already done this. You can advise this is because it is your current residence but also because you need them to update so it will show on the API for Brave.
Sounds like you might be trying to set up with Uphold through Brave. Try creating your account at Uphold.com instead - I think there’s an issue with the Brave link. After you set up your account on the Uphold side, you should be able to come back, connect and log in through Brave.