Connect to your SharePoint Admin center via Microsoft SharePoint Online PowerShell:


Connect-SPOService

type your admin center URL: https://<yourdomain>-admin.sharepoint.com

and login.


Run the command:

Set-SPOSite -Identity https://<Your site collection url> -DenyAddAndCustomizePages $false


Make sure the scripting setting in the admin portal matches what you set using PowerShell, or the site collection setting may be overridden again in the next 24 hours.