Usually, this error code suggests that the serial number on your Wii U is modified. To resolve this, you'll need to apply the correct serial number to your console.

1. Flip your console over and note down the serial ID. It's the number that appears on the white sticker at the top.
2. Install FTPiiU manually or from the Homebrew App Store.
3. With your Wii U booted into Aroma, press L+Down+Select to enter the Aroma plugin menu. Then, go to FTPiiU and set Allow access to system files to true.
4. On your computer, use any FTP client you want (you can even use Windows Explorer) and connect to the IP/port shown in FTPiiU.
5. Navigate to /storage_slc/sys/config/ and download sys-prod.xml to your computer.
6. Open the downloaded sys-prod.xml and ensure that the codes in the code_id and serial_id fields match the serial ID at the bottom of your console. If they don't match, replace them with the correct serial and save the file.
7. Upload the fixed sys-prod.xml file to /storage_slc/system/config, overwriting the previous file when you're asked to.
8. Disconnect from FTP and set Allow access to system files to false. Then, reboot your Wii U and Pretendo should start working once you log into your account.

Thanks @aircookieee for their guide on this here: https://discord.com/channels/408718485913468928/1232274456752422942/1233426788395389049.