In SharePoint Online we are quite used to the fact that custom scripts are denied by default in SharePoint. You can allow custom scripts by using PowerShell (https://docs.microsoft.com/en-us/sharepoint/allow-or-prevent-custom-script). Usually you realize this when you run into permission issues, missing libraries or features or migration errors.
It looks like Microsoft has added this same feature to SharePoint 2019 on-premise server also. We run into this issue while trying to migrate content from Office 365 to SharePoint 2019. Luckily you can also allow custom scripting in on-prem by running following PowerShell script.
$site = Get-SPSite -Identity "https://intranet.contoso.com" $site.DenyPermissionsMask = [Microsoft.SharePoint.SPBasePermissions]::EmptyMask