When you select “Require users to apply a label to their email and documents” inside a label policy in Microsoft Information Protection, users will be required to classify the documents they create/modify.
To add this requirement, access the compliance portal and select a label policy you created.
Then click “Edit policy” and go straight into “Settings”. Select “Require users to apply a label to their emails and documents”, then save the label policy.
Be aware that the policy may take a while to replicate.