The ATP Safe Links feature is part of Office 365 Advanced Threat Protection.
This feature is default included in:
Office 365 Enterprise E5
If your organization is using another Office 365 Enterprise subscription, Office 365 Advanced Threat Protection can be purchased as an Add-on in Customer Portal (Store > Buy Additional Resources)
At the bottom of the this page you find a full list of the Office subscriptions where you can buy this Add-on: https://products.office.com/nl-be/exchange/online-email-threat-protection#howToBuy
It's not possible to buy this Add-on with these subscriptions:
Enterprise Mobility + Security E5
More information about Safe Links: