If you have not put anything into it or define its length. You are going to get an error when trying to use it. The array must contain something or it will error when trying to access it as there is nothing to access as you have not put anything into it.
So if you go and say set up an tarray like so
TArray<YourClass*> Testarray;
The in constructor do this or do not do this and use add() or insert or append()
Testarray.SetNum(10);
At that point you should have 10 Testarray to put stuff into. if you do not use SetNum().
Then use add(); or addunique(); or append() or insert(). and those should work.
Good luck with this.