Windows Defender Security Center

Hi Rshanouda, to your question, no, the trial is not limited in any way in terms of number of machines you onboard.

Likely candidates for this behavior are typically limiting network proxies (easy to test, if possible, put the machine on a direct internet connection and see if it appears).

You can also use the troubleshooting information here - https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-atp/troubleshoot-onboarding-windows-defender-advanced-threat-protection

or this - https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-atp/configure-proxy-internet-windows-defender-advanced-threat-protection

And here's some good tooling and advice on how to check for connectivity issues from Alex -https://www.verboon.info/2019/02/check-windows-defender-atp-client-status-with-powershell/

If the above doesn't help, I suggest opening a support case (top right corner of the portal under the '?" icon)  

Rshanouda 

i will also go and update your Baseline to 1809 so you can benefit from the latest security features

see here 

https://blogs.technet.microsoft.com/secguide/2018/11/20/security-baseline-final-for-windows-10-v1809-and-windows-server-2019/

run it with the desire below options.

.\BaselineLocalInstall.ps1 -Win10DomainJoined      - for Windows 10 v1809, domain-joined

.\BaselineLocalInstall.ps1 -Win10NonDomainJoined   - for Windows 10 v1809, non-domain-joined