We can able to add and dispense the inventories successfully in our OpenEMR (5.0).
Are you getting any error during save the process ? if yes , please share the screen shot of error message .
It would be better if we get more details like,
1.Version of your OpenEMR
2.Screen of the error message
Also check your apache error log to find more detail about the error ?
Let us know if any clarifications.