Hi
Trying to install a add-on called Real-Estate after upgrading to 5.6.1.2. I get the error: 'Could not determine temp directory, please specify a cache_dir manually'

I have also lost my edit bar along the top of the pages in content. When I try and use Real-Estate it just looks blank and reports 'Could not determine temp directory, please specify a cache_dir manually'

I also modified a theme and of course fell into the trap of upgrading and loosing elements I modified. How to I safely modify a them and some core elements?
Kind Regards,
Jamie