Bitdefender Free has not been updated since 2013 (for Windows 8). They appear to not be updating for compatibility with Windows 10 or will be providing any support…
I added 2013 to the topic title to make it more informative.
To clarify even more, below is quoted from the Bitdefender Free Forum:
Guru Poster
Group: Technical Support
Posts: 4,324
Joined: 20-May 10
Member No.: 34,114
Hello,
Bitdefender Antivirus Free Edition is not compatible with Windows 10.
You will be able to install it but there will be side effects including but not limited to inaccurate detection, unwanted false alarms of legitimate apps that come with Windows 10, etc.
As the best protection for the new OS, we would recommend one of the Bitdefender 2015 classic line products: Antivirus Plus, Internet Security, Total Security
We do not offer technical support for Bitdefender Antivirus Free Edition on Windows 10.
I reread the Bitdefender topic that was linked but I see there was no mentioning of version 2013. So I removed the reference to 2013 from the topic title. Sorry for the confusion I may have caused.
For your kind information:
Please note:
It is only the free version that is not fully compatible with Windows 10.
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Correct, but the New Bitdefender Free 2016 BETA does support Windows 10 
Of course, it’s still a beta