Reinstallation of drivers, as mentioned by @ilkhd, worked in my situation. Summary first, details afterwards.
Summary: Ensure you have a cleanly installed set of video drivers. In some cases, revert to the original set provided by the OEM, or the original parts providers, in order to ensure that all parts are working together in the right way in order to provide the correct HDCP verification. Check that the versions display correctly when verifying using different tools and that all system drivers work together. In my case, a combination of incorrect/incompatible newer driver installations messed things up. Fixing the default video driver installation re-enabled the capability to play HDCP content.
Details:
In my case, iTunes on Windows 10 running on a laptop of 2015 vintage. Laptop is a Dell with display port capability, though in this case through a dock and connecting to external monitors, one through the display port, the other through VGA. That said, the main need is to be able to display purchased content, like movies, on iTunes on the built-in laptop display. iTunes would error out notifying that the system must either have a built-in display or be connected to a display that has hardware HDCP support, like an HDTV, in order to play HD content. This didn't make sense b/c the laptop HAS a built-in display that is 1080p. Why does it think that it doesn't have this display and/or have HDCP capability?
While looking through the driver details in Device Manager>Display adapters for the Intel graphics, I noticed what looked like an Intel HDCP service executable in the driver list. It is at C:\Windows\system32\IntelCpHDCPSvc.exe. So it seemed that the support for HDCP was there, but somehow not active.
Then I noticed that the driver version wasn't displaying consistently in System Information and from various update tools that scan the versions, such as Dell Update and Intel Update, and even the device manager dialog for the Intel Graphics. Tried a driver update for the Intel graphics. Did not fix the consistency issue and the driver version displayed in some places didn't match the driver version I thought I installed.
Then, I rolled back the intel driver. After doing that, the driver versions displayed consistently, and also showed the version that I had just installed. Go figure.
Tried iTunes again. Got ALMOST the same message. Instead of saying that I need a built in display, it said that I need to move the window fully into the built in display, or to display externally to a TV or other HDCP capable device. The window was on one of the external displays. Moved the window to the built in display and... it works. Content plays, no error message.
