are there unmanged clients? with no SEPM?
Managed clients are automatically licensed if the package is exported from SEPM with a paid license or upon first heartbeat cycle.
In the installation drive inside SEP folder there is already a license file sep_NE.slf which is a license file for 1 machine and with no expiration date. It will apply automatically to the SEP unmanaged client.
Here is an extract from the license file:
<serial_number>Mxxxxxxx</serial_number>
<product_name>Symantec Endpoint Protection 12.1 Inbuilt Never Expires</product_name>
you have installed the client has an unamanged client so follow the steps below to install the license
License it as per this document
Licensing an unmanaged Windows client
https://support.symantec.com/en_US/article.HOWTO81071.html