Sometimes won't take a picture

Your best answer might come from another A71 owner, not me Google turns up some Samsung user forums, and possibly find a PDF manual for the phone as well. Typically cameras will not shoot if focus is not detected - depending on camera settings there is usually a focus box that turns green. Because the problem is inconsistent, that seems likely. You might change the Autofocus setting to Continuous focus or screen Touch focus if available.

SImilarly, as you note, especially in multi-shot HDR (e.g. low light) modes the camera must be held still and the shutter finger may shake it - often there's an on screen hand shake warning.  I like to set a 2 or 3 second delay when hand held to steady the phone.

If the app is glitchy or some user setting is causing trouble, a good cure is usually to go into Android Settings, locate the app and empty its caches and data - this will restore out of the box default condition. Restarting the phone after that can help.

If there's a hardware issue, you might confirm it by installing another camera app from the Play Store - Open Camera is free and works on most Android cameras, but these days it's difficult to recommend any third-party app for any particular model because of the plethora of lens and sensor configurations. If a second app has the same problems that suggests a mechanical issue and basis for returning the phone.  You might find a working third-party camera app you like better than Samsung's stock app - I've used the same apps over several phones, am familiar and rarely bother with the manufacturer's preinstalled ones.