cannot take screenshots in a banking app I have an unlocked Android 6.0.1 device, and I've installed a couple of banking apps.

For obvious security and paper trail reasons, I'd like to take some screenshots of my interaction with my bank.  However, somehow the low-volume+power buttons no longer takes any screenshots whilst in either of the banking apps.

Why?  How do I disable such a nuisance?
This is a result of FLAG_SECURE. The reason is obviously again security - other apps can either leverage the native screenshot API, or just read your captured results from storage to potentially obtain sensitive info.

If you're already into Xposed framework, then there are a bunch of modules that will disable the check for the flag system-wide, such as this one.

As @iBug already pointed out, always exercise extreme caution when working around security measures, especially when a 3rd-party app is involved.