To apply a field security policy, you must activate it.
Steps
Go to Dynamic field security management > Policies > All field security policies.
In the list, find and select the desired field security policy.
On the Action Pane, click Version.
Click Activate.
Notes
Only one version of a field security policy can be active. So, if you have a version active and you activate another version, the previously active version is deactivated.
Make sure to not create and activate another field security policy with the same tables in the table setup.
You can manually deactivate an active field security policy version.
You cannot edit an active field security policy.