Tried with Ubuntu 14.04 and PHP 5.5.9 and I got the same error

Same result on Ubuntu 18.04 PHP version 7.2
Undefined index: g-recaptcha-response in /var/www/html/index.php on line 312

Same result on windows PHP version 7.0
Notice: Undefined index: g-recaptcha-response in D:\Work\BlockChain\piWallet-master\index.php on line 312

Same problem Ubuntu 18.04
Notice: Undefined index: g-recaptcha-response in /var/www/html/index.php on line 312

i had this issue as well but.. it has to do with using a different version of a php plugin. what fixed it for me was

sudo add-apt-repository -y ppa:ondrej/php
sudo apt-get update
sudo apt-get install git apache2 php5.6 php5.6-mysql php5.6-gd libapache2-mod-php5.6 mysql-server

you probably already did this once. but along the way an apt-get upgrade or apt-get update probably messed it up for you like it did for me. Not a solution to it running on php5.6 but it will get it going for ya. hoepfully... maybe. ;)

Ever find a fix for this? I have same problem.

i have the same problem the g_recaptcha_response seem to be empty, i try to use on more than one page my recaptcha and i obtain this error with second page i use it

i have passed through this error, my form was in table and i got the same error than you, i've put all my form and fields between some

between some div, no table, just div