Here is a great video on elbow extension in relation to hand balancing by Yuri Marmerstein. If you can’t lock out your elbow while in a handstand, this video will give you everything you need to fix this somewhat rare issue. This may not be a big problem in the beginning of your hand balancing training, but once you progress and try to achieve something like the one hand handstand, it might prove to be a bigger issue than you thought initially.
Some people simply have a psychological barrier which prevents them from fully extending their elbows in a handstand. On the other hand, some literally cannot straighten their arms fully, whether due to tension in biceps or some other physical factor. Yuri Marmerstein shows two different workouts you can do at your home to work on this issue gradually in order to fix it permanently.
Also make sure to check out The True Art and Science of Hand Balancing by Professor Paulinetti and Robert Jones, which is a great resource for anyone who’s into hand balancing.