SharePoint Server
A family of Microsoft on-premises document management and storage systems.
1,637 questions
Hi, anonymous user-7489 ,
For now, we still can use SharePoint Designer 2013 to create custom list forms in SharePoint 2016. I tested with the latest patch for SharePoint 2016 and new custom form still works. It is SharePoint Online which has shift to using PowerApps for custom forms and remove this feature from April 25, 2020.
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Hi, anonymous user-7489 ,
Thanks for your update. What kind of error stops you from creating custom forms in, like you called, Top-level site collection A and the subsite 2? Does this issue occur on all the lists in the sites even with newly created lists and any content type? Are the lists with issue using certain lookup columns?
And I will suggest you to open a new thread for this issue with all the information you know, you can answer the questions there. A new thread will attract more community experts for it. It is not quite easy to find the actual problem in a comment.