Hi Tiara,

Sometimes, one module need another module as dependency to run well.

I've checked in my Odoo 12, Purchase app need Accounting app (Invoicing part) to be working . If you want to double check, you can enter debug mode and go to app you would like to check. Later on, in technical tab, scroll down until you find dependency.

You also need to check another custom addons you added before (if any), because it might also use accounting app as it's base dependency.

Best regards,

Altela (altelasoftware.com)