There are multiple ways to install secure folder on your Tab A7. Just search 'how to install Secure Folder on Tab A7 (model no.)'. Then, you should look for some reputable YouTube tutorials, as well as websites such as xda developers.
Another way is to go on Sammobile.com and go on the tab that says 'search for Samsung apks' or something similar to that nature. Then, simply search for the secure folder app and install it.
Another way is go to on websites, such as apkpure, an1, etc. and search for the app on there. Just check to see those apps don't have viruses before you install them.
Moreover, the final way is to possibly use Aurora store (download the app from the internet), spoof your device to one that IS compatible with Secure Folder and the install it :).
Hope this helps!