Windows Defender - Virus and spyware definitions couldn't be updated

When did Defender update?  What definitions are installed? Remember Defender should autoupdate appx every 24 hrs as needed.

https://www.microsoft.com/security/portal/definitions/whatsnew.aspx

See Troubleshoot Windows Defender Update Issues in Windows 10

Any other security program installed?

The behavior you are describing may occasionally occur if another antivirus program is installed or there are remnants of a third party antivirus program still installed

Go to Security and Maintenance, look in 'Security' and see what program is listed under Virus Protection.  Windows Defender will not be enabled and functioning properly if there is a program other than Windows Defender listed.  Suggest you uninstall any other AV programs on the computer and run the software removal tools for any security software (AVG, Baidu, Kaspersky, McAfee, Norton, whatever) that may have previously been on the computer (including a trial version of McAfee, Norton, Trend, Webroot, etc that may have been on the computer when it was new) followed by a restart of the computer. See List of anti-malware product removal tools.

Alternatively - - -

You may prefer to install a third party antivirus program which should disable Windows Defender.

There are a number of viable free and paid options available – take your pick…

Choosing an Anti-Virus Program

Is Windows Defender The Best For Windows 10?  Good article but ignore the ads.

http://windows.microsoft.com/en-US/windows/antivirus-partners#AVtabs=win10

Regards…

http://blog.emsisoft.com/2015/01/27/top-10-ways-pups-sneak-onto-your-computer-and-how-to-avoid-them/

I am having this problem too. This is a new problem.

No it has happened before but it cleared its self!

why can't i update virus definitions?

Same issue today 2017-09-29 @ 10:50 BST