App downloads however does not install

[denzelday]

I downloaded an app from Internet because since marshmallow update my Sony Xperia Z5 no longer let's me download artist and song information all my songs reset to the original art work of when I downloaded them. I allowed unknown source download and successfully download the app "music" with the extension of album art and artist info download by Sony however my Z5 everytime I try to install the app; loads it and notify me with the following message "App not installed." this error also happens to a few other apk apps I downloaded. I solved this issue when I had lollipop and first got the phone before the marshmallow update and cannot remember how I did it.