Do you have the asset id (KUID) number and name of the asset associated with this error? That information is really helpful when answering questions about stuff like this.
My hunch, however, says it's related to Speed Trees and is caused by loading older Speed Trees because the older Speed Trees, meant for TS12, are not compatible with T:ANE and up including TRS19.
With that said, there is no way to repair the problem yourself, and all you can do is replace the trees, or shrubs with a version that is compatible. Unfortunately at the present time we can only replace the faulty assets manually so it means opening the route up for editing in Surveyor and manually replacing the missing trees. Once done, you then can run the Delete Missing Assets to remove the reference to the defective assets.
