cant really comment without a link where you are experiencing this issue

all i get with that code – at the moment – is
[108] weekday:code currently in-active

which means exactly that. I cannot see the issue you are describing (but i assume that would need a code that – in theory – should be valid) ….

Hmm, distinctly strange.
when the check returns ‘OK’ it should simply just reload the page (which it does not in your case for reasons we have to find out)

— before doing any of the below it would probably be a good idea to make a backup —

first thing to do : enable debug like so https://www.wp-pizza.com/topic/how-to-enable-debug-in-wordpress/ and see if anything appears in your debug.log
(although to be honest i think this is a js issue not a php issue)

second – temporarily at least or in a test environment –
disable ALL other plugins (apart from th emain wppizza plugin and the coupon code plugin)
if it works then, re-enable one by one (and let me know which one causes the issue)

third – of above does not work, not only disable all other plugins , but also use a default theme (like 2016) and see if it’s in the theme somewhere (as it would be if it works with a default theme)

4th – if none of the above make any difference, please install https://en-gb.wordpress.org/plugins/duplicator/

and contact me again here (we’ll have to go into more detail then)

you should most certainly – first of all – fix the “sending QUERY packet” error
http://stackoverflow.com/questions/30753674/error-while-sending-query-packet

although i do not think this is caused by wppizza actually, if anything makes your db connection fall over it will of course impact everything (just setting the max packet size to something higher than default will probably do the job there)

regarding the actual issue, i cannot really test/see anything as you seem to have changed validation and/or codes

nor have you told me if disabling other plugins/theme fixes the issue or not , so i cannot say anything else really