Save a File to /AppInventor/assets

Browsing the forum reveals some trouble/issues with it. At first I had difficulties as well. Therefore I share my blocks with you. May help you. With it I can store a file and I do see it within the phone's directory. I use a Samsung Note 4/Android 6.0.1:

the assets of your app are read only... therefore you no really can write there

what you are doing in the example is to store to directory /AppInventor/assets... this directory is used by the companion app on devices < Android 10 to store the assets... so your example actually is not helpful for others

others should follow the example by @Anke here to store a file to ASD