Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

Thanks for reported, it seems like that is bug if you didn't choose any plan for this custom details section, then it will hit this warning error.

By the way, to avoid this error, you have to choose at least 1 plan if you do not want to apply this to all the existing plans from your site backend > Payplans > Orders > custom fields > edit custom field .