I'm just confused as to why XBL achievements can be unlocked in other Game Pass games like Gears of War 4 and Wolfenstein II, but not on Reach.
Because of Easy Anti Cheat. Can only be ran in Anti-Cheat disabled mode which in turns disables achievements and allows mods to be ran
XB Game Pass version and I'm getting the "failed" error when activating the trainer. Tried the included launcher shortcut from the trainer zip and "Halo:MCC Anti-Cheat Disabled (Mods and Limited Services)" shortcut. Disabled MS Defender AV and FW and launched both Halo and trainer as local admin.
19:55:58: Failed attaching to process. Very likely the game is protected by anticheat software. Please check out the trainer notes to see if there are any special instructions.
EAC is likely still running. You have to run the game with EAC disabled. Ask Caliber to make a video on how to do this manually if the included shortcut is not working for you.