Unable to activate Script editor for a classic Wiki site in SharePoint

I cannot activate the Script editor for a classic Wiki site in SharePoint.

What I did until now:

Under Custom Script selected: "Allow users to run custom script on personal sites" and "Allow users to run custom script on self-service created sites".
Enabled "SharePoint Server Publishing".
Enabled "SharePoint Server Publishing Infrastructure".
Ran "Set-SPOSite <SiteURL> -DenyAddAndCustomizePages 0" in PowerShell.