Extension install failed, now cant get the functions to work or reinstall successfully. How to fix please?
Some extensions don't fully clean up behind themselves when they are uninstalled causing problems when you attempt to install again.  You will need to look at what the extension is trying to do when it installs and find what is failing.  If the fields/options etc already exist you can comment out that part of the installation process.  Alternatively, remove them - but if possible use the API rather than direct SQL to reduce the chances of leaving things in an inconsistent state.

If you can't install, uninstall and then reinstall an extension cleanly then it is worth logging an issue in the extension's issue queue.  I have just tested that sequence with the GDPR extension without problem.

And please get your 4.7 system upgraded!