Ever since the AIO update I have never been able to add the bugfix-merge without it crashing on load due to parsing errors :_:
you haven't installed bugfix-merge according to that error message.
download the latest AIO, make sure you are on the bugfix-merge Branch and download that branch, it contains both bugfix and te merged mod (mod folder will be gone)
unzip the aio into folder, take the game folder from bugfix-merge and copy it so it overwrites game folder.
go into the game folder and delete scripts.rpa and gui.rpa.
now be for warned it i in alpha, lots, like all of mod folder code has moved. and Mod was at least the size of the original game. if not larger.
any traceback with a properly installed bugfix-merge will have a 51.4 alpha as version.
installing bugfix-merge and then installing bugfix and dev will live the game in an extremely broken state. same as installing bugfix dev and then bugfix-merge. will leave the game in an extremely broken state. the first, trace errors will still show 51.3.1, the second will show trace errors with 51.4 alpha but won't be playable.
also be ready this will break saves, sometimes in the updates in the same day, but for certain between 51.3 to 51.4