Have a specific error not covered? Boot into GodMode9, check your key slots, and cross-reference with the official 3DSbrew wiki. The answer is almost always a missing slot0x18 key.
To obtain a legitimate aes_keys.txt , users typically use a custom firmware tool like on a physical 3DS to "dump" the internal keys into a text format. Alternatively, many users bypass the need for this file entirely by specifically seeking out "decrypted" game versions. 3ds aeskeystxt work