For anyone having trouble installing Adastra, i had similar issues with downloading and installing MacroDroid Helper, which is an app that allows automated tasks on my android phone. Their provided solution was to use ADB (plug your android device into the computer) and do this command:
/adb install --bypass-low-target-sdk-block [apk name]
This works for adastra too. It allows apps for previous android versions to be able to be installed.
Hope this works!