You should be able to browse to the X-Plane 11 folder and tell Traffic Global to install there, even if it thinks it is elsewhere. However, this does mean you need an up-to-date version of X-Plane as earlier versions may not be recognised. Also, it may be that, originally, X-Plane 11 was installed on a drive which died or was removed/changed and X-Plane 11 was installed on a new drive letter without being able to uninstall the previous installed version.
Either way Traffic Global reads the X-Plane install log to see where XP11 is. It usually selects the latest entry but if there are a few (where X-Plane 11 may have been installed in different locations in the past) it can cause problems with the installing knowing where X-Plane 11 is actually installed.
To resolve this, search the machine for this file:
You may have a few entries in there so you can just delete all the locations apart from the one where is X-Plane 11 is currently installed. This will enable Traffic Global to navigate to the current, correct, location.