Logout, backup your MasterMerchant SavedVariables files, delete them in the SavedVariabls folder.
Dlete total folder AddOns/MasterMerchant.
Reinstall it new.
Update needed libraries/dependencies for MM and install them as standalone versions like addons directly into the AddOns folder!
Login and try again.
If the error still appears, use the offical MM comments here (
https://www.esoui.com/downloads/info...rMerchant.html) and describe how to get the error, so it can be fixed.