Country Mismatch
Short answer how to fix = Change your passport/ID on custodial partner to match the country you chose for Rewards -OR- go to Hamburger Menu 
→ Rewards → Reset your Rewards. Then choose the country your passport/ID is verified with/
Full explanation and solutions:
If you get the Error: Country Mismatch message then this means the country you chose for Brave Rewards is different than the one you have Verified with using a custodial partner. Verification purposes relies more on your passport or other photo ID than it does for your address.
To use a random example, let’s assume you originally created your account while in Poland and you submitted your Poland passport. Everything was fine and you’ve used Uphold for years. Now you move to France and update your address with Uphold to show you France address. You also choose France for your Rewards country. When you go to connect to Uphold, you’ll get the Country Mismatch error because Uphold is reporting to Brave that your passport is from Poland.
How do you fix this? You either have to update your passport or photo ID to be one from France or you have to go to Hamburger Menu → Rewards → Reset your Rewards which will start Rewards fresh. You’ll then choose the country you verified with (in our example, would be Poland), and you’d then be able to link to your custodial partner (such as Uphold).
NOTE
I know for many from EU, this can be frustrating as you don’t need to update passport or ID when you change countries as long as it’s within the EU. In fact, getting resident permits sometimes aren’t possible and everyone having a driver’s license might sound quite odd. Unfortunately, that passport or ID is what it’s based on for now.
So if you can’t get one for the country you’re currently in, then you’ll just not be able to select your current country or you won’t be able to connect Brave to a custodial partner. This is something that Brave is working with custodial partners to improve.