If any OU (organisational unit) is deactivated in CSoD, it will not be available in Training Plans.
In a similar way, if a parent OU is inactive in CSoD, this OU and its child OUs will not be visible in Training Plans.
For example, if you have the following OUs configured: Europe (parent) > Portugal, Spain, France (children) —» if Europe is deactivated, then the child OUs will not be available in TP.
OUs can only activated and deactivated in CSoD.