Hi digitaldiego007
Look at the menu item for page giving 500 error and verify that it's using your template
Also are you sure that you have only JA Simpli installed as joomlart template ?
Because ja simpli do not use JAT3 Framework so it is not needed any JAT3 Plugin
The template is not based on any framework. It's clean and simple for quick implementation & comes loaded with a powerful admin backend for easy customization. No coding required.