Note: I am not responsible for bricking your Wii U.
This guide is for Tiramisu or Aroma Users
Here on this post, I'll be giving solutions on how to fix these errors below.

1. Couldn't find NUSspli file on the SD Card
Tiramisu:
Make sure you have NUSspli.rpx in sd:/wiiu/apps/NUSspli/ .
Aroma:
Make sure you have NUSspli.wuhb in sd:/wiiu/apps/ .

2. Unsupported Environment
Switch to an environment that NUSspli supports, if you are booting NUSspli from a supported environment but is still having this error, follow these steps for your environment.
Tiramisu:
1. Go to https://tiramisu.foryour.cafe , click on Download Tiramisu, extract and copy the contents of the .zip file to the root of your sd card overwriting existing files, reboot the Wii U and try again.
2. Switch to a different environment that NUSspli supports and boot the one your environment uses.
Aroma:
1. Switch environment

3. snip

This is all the errors that I could solve. Any other errors that are not listed here can be fixed if you ask on NUSspli thread or on GitHub.