Hi @Abhishek Shrivastava ,
“Site pages” are essentially a ‘document library’. So it can't prevent users from creating it.
As a workaround, you can Stop inhering the group's “File Operations” permission on the “Site pages” library and grant them “Read” permission so that users cannot create pages.
Problems with document library went in classic mode, modify “File Operations” permissions. Under Site permissions, check “Use Remote Interfaces - Use SOAP, Web DAV, the Client Object Model or SharePoint Designer interfaces to access the Web site. (Deprecated)”->Submit
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