With SAP S/4HANA 2021, the usability for activating the trial period for SAP Application Interface Framework has been improved.
How to activate the trial period so far?
SAP Application Interface Framework (AIF) is an integral part of SAP S/4HANA, however if you like to create your own AIF interfaces, an AIF license is required. For a limited period of time, actually 90 days, you can try out the complete AIF capabilities including customizing and monitoring & error handling. So far, the trial period has been activated implicitly the moment you created your first AIF interface within your own custom AIF namespace. So, it might have happened that you started the trial period by mistake.
Note: For creating variants for SAP delivered AIF content in order to add checks or overwrite mappings, you do not need an AIF license. Since variants need to be created within your own AIF namespace, the license check isn't applied to the creation of namespaces, and hence the trial period only starts when creating an interface.
What's new?
From SAP S/4HANA 2021 on, you can explicitly activate the starting time of the trial period, i.e., you can decide on the starting time on your own This is done using the
Switch Framework.
Let me show you how this works.
Run transaction
SFW5. A dialog comes up informing you about the risks of activating irreversible business functions. Click on
Continue.

In the switch framework, expand the Enterprise Business Functions folder, and select the checkbox of the
AIF Switch for trial period (/AIF/TRIAL_PERIOD), then click on the
Activate Changes button.

In the upcoming dialog, you see a caution that the current settings cannot be restored. Note, that the trial period activation is irreversible, i.e., you can't deactivate and activate again in order to prolong the overall trial period. If you are sure that the trial period should start now, click on the button
Activate Changes.

This will schedule a job which runs the activation in the background.

If you confirm the dialog, the planned status shows that the business function will be activated.

The display of the status won't be refreshed automatically. You need to leave the transaction and enter again to see the updated status. Once the activation went through, the planned status shows that the business function will remain activated.

Now you can test AIF for 90 days.