As it was sold, the Galaxy J5 was powered by the Android 5.1 operating system. The NHS Covid-19 app can't be used with that version of Android, as it requires version 6.0 or later. So that might well be the cause of your problem.
However the operating system on a Galaxy J5 can be updated to version 6.0.1, which the app should then work with. (It works on my Alcatel phone, which uses Android 6.0).
So check which version of Android your phone uses and, if necessary, update it:
https://www.helpforsmartphone.com/public/en/samsung/galaxy-j5/android-5-1/guides/23/Update-software-Samsung-Galaxy-J5