That version is 2 years old... maybe is time to plan an upgrade !
From the 18.104.22.168 release notes:
In this release of 6.0, the following licensing changes are available:
- You do not require a license file for using Manager/Central Manager version 6.0.7.x or above. Therefore, the
Licenses tab under the Manager node has been removed.
- No license file is required for enabling IPS on I-series and M-series Sensors; no license is required for enabling NAC
on N-450 Sensors. In other words, when you add a Sensor to the Manager, upon discovery, the native functionality
supported on the Sensor model is automatically enabled.
- Like in earlier releases, you require an add-on license to enable NAC on M-series Sensors. Earlier, you could
import the add-on license under Manager > Licenses tab. In this release, you can import/assign the license using
the Device List > Add-On Licenses page.
- With this release, the Manager will not raise any fault on Sensor license expiry.
Refer to the Release Notes of the Manager version that you are installing or upgrading to for the latest information on
So as you can see you no longer need a license file if you upgrade your manager to 22.214.171.124 or above.
You can upgrade to 126.96.36.199 from 188.8.131.52 without problems, but i would suggest that you upgrade to the latest 6.1.1.x version if possible. If you download the release notes for each of the builds, you will see a table almost at the end of the document that indicates which is the minimum manager version you need in order to move to the one you want to install. In this way you can work out your upgrade path.