It is a issue with the version of MySQL that installs on Ubuntu 16.04
I had to change one of the tables from INT to VARCHAR this allows to add credit but then it breaks something else along the line.
Quoting from a post I found online: “parent_uid colum by changing the type (from int to string or varchar) and to accept empty string (NULL checkbox checked) or using 0 as default value”
Before doing that you could try:
Try modifying mysqld.cnf (I would suggest making a backup of the file first)
nano -w /etc/mysql/mysql.conf.d/mysqld.cnf
and add below to the end of the file: