Hello
It seems a conflict with any other plugin. A similar funciton/class/library in two different plugin can cause such kind of problem.
To get recovered from it, follow the steps below:
1. Access the plugin folder from cPanel/FTP
2. Rename WP ERP into something. (Like: WP ERP1)
3. You’ll be able to access the site admin panel now. Go to the dashboard->plugins and disable the other plugins temporarily.
4. Activate WP ERP.
5. Now, enable the other plugins one by one.
6. The last plugin you have enabled before getting the same problem is conflicting with WP ERP.
I hope this helps.
