Hi @Moheed Gillani ,
This is by design in SharePoint Online. Lot of setting are disabled by default in SharePoint Online. Keeping in mind security considerations, SharePoint admin center->Settings->Custom Script is turned off by default. This is done to disable some of the customization features as well as to disable couple of other features that depend on script. SharePoint Designer Customizations are also disabled by default.
For more Information:
If the issue still exists, we suggest you can create a new service request ,and more professional will help you.
(Microsoft 365 admin center ->> Support ->> New service request)
Thanks for your understanding.
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