actually, i’m not even certain if you are talking about admin or frontend here

forget that – i did not read this properly.
in any event , i would need admin access to have a look what happens when adding a meal size
if you post it here, mark it as a private reply

thanks:
your problem is javascript errors that are either thrown buy your plugin or theme

Uncaught Error: Syntax error, unrecognized expression: a[rel=]
Uncaught TypeError: Cannot read property 'hasClass' of undefined

you should de-activate all other plugins and use a default theme to track it down where those errors come from (as they are all thrown in together in a load.scripts.php i cannot tell you which one is the one likely to cause the problem)

that said, my initial suspicion would be your “jquery-updater” . i cannot even begin to tell you why it is a very bad idea to replace the jquery version that comes with wordpress that way
(but here’s a great article i fully agree with https://pippinsplugins.com/why-loading-your-own-jquery-is-irresponsible/ )
It may be the case that that plugin is not to blame , but de-activating it might at least make debugging the underlying issue easier

you should also enable debug like so https://www.wp-pizza.com/topic/how-to-enable-debug-in-wordpress/ and check your debug log

in short, what you want to do is:
turn off all plugins. keep theme.
can you use quickedit ?
if not, switch theme to a default theme. can you use quickedit now ?
if you can, speak to theme developers to fix it