How can I change directory of AnkiDroid to external Storage
The reason you are not able to store an app on your memory card could be using a card formatted in "Portable storage" mode. In that case no one single app can be pushed out of internal memory, but reformatting of a memory card in "Internal storage" mode can change that. Another reason could be a restriction made by app's developer and there is nothing a user can do about it.
Sources: https://www.androidpit.com/how-to-format-microsd-cards-as-internal-storage
https://developer.android.com/guide/topics/data/install-location.html