Some platforms like android do not give you direct access to the streaming assets directory. On android the streaming assets directory is compressed into the apk, and so is not available for you to just read/write files to normally.
It is advised that if you want to save/load files on mobile devices, you should be doing so to the persistantDataPath instead, as you should have no problem reading/writing files to this directory.
Unity provides source code: https://learn.unity.com/tutorial/working-with-the-streamingassets-folder-2019-2#5ff39515edbc2a4fd922da98
– Chris-Ender