Unless you're a developer you should never modify anything in the Master Language or Master Style. This suggests you're running a site with Debug mode enabled, ideally Debug mode should be always disabled and only enabled if it is required for a specific task.
You should change the text in each language that requires the change, e.g. English (US)
I tried to change it to- "Please UPLOAD a file or CHECK Does Not Have a File Option." and when I re-look in admin, this is what it appears but when error comes in XF RM, different phase TEXT.
Using only English language.