i *might* be the case that you simply have some parameters not set /saved (i believe you had some “fatal error- out of memory” messages in your logs)

so i would suggest (short of reinstalling everything of course)
you go through all the wppizza admin pages (sizes, settings, gateways etc etc ) as well as all your wppizza plugin pages (add ingredients-> options, add ingredients-> settings etc, delivery by postcode options/settings etc etc – you get the idea)
and just save them once so we know you have no “missing” parameters in your db

I cannot guarantee this fixes it, but certainly worth to make sure the problem is not there (should only take 5 minutes to do really)

simply ignoring “session_start(): Cannot start session..” is not a good idea . might work seemingly but you will probably end up with issues at some other point
also, fixing this will most likely also fix all other warnings/notices you have there , as they are most likely related

if everything fails, make/send me a duplicator package (put it somewhere i can download it or – if that works for you – simply give me admin access to your site i can do this myself)
(https://wordpress.org/plugins/duplicator/)
and i can install a clone here and see what the outcome is in my scenario