“Undefined Index: “ error happens when you submit a form with long input fields or many rows in it.

error while entering more than 70 item

when i create purchase and add 70 items only 52 is saved other data is lossed

Solution 1: 

Increase the max_input_vars variable in your php.ini file.  For technical details Refer to official PHP documentation
There are a few possible ways to do this:

1. Edit the php.ini file and change the value of max_input_vars to something higher like 5000.
2. Or change it from .htaccess file by this statement: (more details here)php_value max_input_vars 5000
3. If the above 1,2 didn’t work then contact your hosting provider to help you change the ini file variable.