add some AA troubleshooting FAQ

Author: arcoraven

apps/portal/src/app/engine/troubleshooting/page.mdx

@@ -93,4 +93,31 @@ Even though the transaction is expected to consume less gas, the transaction wil
 
 **How to resolve**
 
-Add more funds to your backend wallet. Or wait until the gas fees on the chain are lower and try again.
+Add more funds to your backend wallet. Or wait until the gas fees on the chain are lower and try again.
+
+## Sending UserOps
+
+UserOps are **User Op**erations sent by a backend wallet on behalf of smart accounts, used in the Account Abstraction pattern.
+
+### Received "Invalid UserOperation signature or paymaster signature" error
+
+*Also known as: code -32507*
+
+**What it means**
+
+The backend wallet you are sending the transaction with does not have sufficient permissions to send a userOp for this account address.
+
+**How to resolve**
+
+Confirm if the backend wallet is a valid admin for this account.
+If you are using Session Keys, confirm they are not close to or already expired.
+
+### Received "AA21 didn’t pay prefund" error
+
+**What it means**
+
+Your thirdweb API key used by Engine is not properly configured to use Account Abstraction, or you have a [sponsorship rule](https://portal.thirdweb.com/connect/account-abstraction/sponsorship-rules) that has been exceeded. 
+
+**How to resolve**
+
+Confirm if the API key is tied to an account with valid payment. Also confirm if there are any sponsorship rule for the Engine API key.