Unable to mount Windows 10 partition; it "is in an unsafe state"

You need to disable the "Fast Startup" feature. To quote the How-To-Geek:

Open up your power options by hitting Windows+X or right-clicking your Start menu and selecting Power Options. In the Power Options window, click “Choose what the power buttons do.”

If this is the first time you’ve messed with these settings, you’ll need click “Change settings that are currently unavailable” to make the Fast Startup option available for configuration.

Scroll to the bottom of the window and you should see “Turn on fast startup (recommended)"

Uncheck the "fast startup" box.

Then, shut down Windows 10, and you should be able to mount the NTFS partition from Ubuntu just fine.

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^{How-To-Geek quotation taken from here.}

Also with disabling fast boot, you also need to disable hibernation.

In an elevated command prompt window, type powercfg /h off.

You have to disable "Fast Startup" in Windows 10 Pro to solve this problem!

You can do this by going to "Control Panel -> Hardware and Sound -> Power Options -> Choose what the power buttons do -> Change settings that are currently unavailable"

Then scroll to the bottom of the window and uncheck "Turn on fast startup (recommended)" box. After that shutdown Windows 10 Pro. Then boot into Ubuntu. You will find that Windows partition(which is NTFS type) gets mounted easily !