Hi @サンデャ 管理者 ,
According to the research , you could enable add and custom page by using PowerShell command.
- Login to your SharePoint Farm SharePoint 2019 On-Premise
- Open SharePoint 2019 Management Shell as administrator
- Execute the following PowerShell command: (Get-SPSite -Identity "https://your-host/sites/modern-site").DenyPermissionsMask = [Microsoft.SharePoint.SPBasePermissions]::AddAndCustomizePages
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