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.

How-To-Geek quotation taken from here.