This was reported in the Reddit sub about two months ago but does not seem to have been reported here. Whenever I open any of my widgets in the editor I get a message saying “Missing Requirements: Post Notifications” with a FIX NOW link.
In the Kustom settings under Notification Mode it says “Automatic: use a customizable notification when battery optimized”. I have battery set to unrestricted for Kustom, so as I understand it, a persistent notification is not needed, and the editor should not bug me about this.
I have fixed this as follows: in Kustom Widget app settings “Allow notifications”, and then under “Notification categories” disable “Kustom Notification”