SharePoint Designer functionality for creating custom Forms within SharePoint Online is deprecated as of April 25, 2020, This change only affects the action of creating new Forms and does not impact the ability to edit previously created Forms. Users who attempt to create new Forms within the Designer will be shown an error message “Could not save the list changes to the server,” when creating new Forms.
Creating Forms using SharePoint Designer is a legacy feature and we recommend users who were leveraging this functionality to now use PowerApps for this purpose.
PowerApps is an easy and powerful tool that allows users operating in the SharePoint Online Modern experience to create and edit custom Forms for SharePoint lists and document libraries right from a browser window.
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