If someone who has access to that system is comfortable with using a command line, this may be the "quick fix":
Otherwise, there are more manual (but graphical) instructions here:
(The reason why you're seeing this error in the first place is because ISRG Root X1 is only included in macOS 10.12.1 or newer, but that system is slightly older than the cutoff. Your other systems probably are all running a newer version of macOS, so they don't have these issues. You could also try updating the affected system to a more recent macOS, instead of the above)