Here are the rich Trading Servers in Adopt Me! Note that players usually go to the neon cave (under the bridge) to trade. If you’re not able to join them then there may be 2 reasons for this.
Go to “settings”, “privacy”, scroll down and make sure “Who can make me a member of their private server?” is set to “everyone”.
If this doesn’t work, your account may not be 13+. To see if your account is below 13, go to “settings”, “Account Info” and scroll down to “Personal. If your account is below 13 you will not be able to change your age, but it will automatically update on your 13th birthday.
