Hi @Karl Finlay ,
To allow custom script in admin center may take 24 hours to take effect. You can try following command in powershell if you don’t have 24 hours to wait.
Connect-SPOService -Url https://[yourtenant]-admin.sharepoint.com -credential admin@[tenant].onmicrosoft.com
Set-SPOSite -Identity https://[yourtenant].sharepoint.com/[siteurl] -DenyAddAndCustomizePages 0
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