Hi, It looks like you purchased the eStore plugin so not sure why you are trying to install the simple shopping cart plugin?
Anyway, basically PHP session on your server is not working properly (it happens when the hosting provider gives you a misconfigured server). Some more details here that should help you chase your hosting provider and get them to fix your server configuration:
https://support.tipsandtricks-hq.com/forums/topic/php-session-not-working-correctly