To make changes to Azure Defender settings, you need to be an owner, contributor, or have the Security Admin role on the subscription.
Sometimes, clearing the browser cache can resolve issues with saving settings in the Azure portal.
You can also use Azure CLI to disable the Defender plan. Open the Azure Cloud Shell or your local terminal and run the following command:
az security pricing create --name "SqlServerVirtualMachines" --tier "Free" --subscription "<your-subscription-id>"
If none of the above steps work, consider opening a support ticket with Microsoft. There might be an issue specific to your subscription or a broader service issue.