Hello,
Please refer to the information in the link below:
MS Defender VPN won't successfully turn on. - Microsoft Q&A
Also please check subscription status. Verify if your Microsoft 365 subscription is active and if the VPN service is indeed included in your subscription plan. Ensure that your Windows OS, Microsoft 365 apps, and any associated services are up to date. Sometimes, known issue can be fixed with an update.
Best Regards,
Hania Lian
============================================
If the Answer is helpful, please click "Accept Answer" and upvote it.