This is somewhat of a annoying problem. Some Hardware cannot play 6.1/7 channel, but there is no easy way right now to solve it properly.
One thing you can do is set the number of channels in the Output Format DSP to your actual speaker configuration, for example 7.1, then it'll convert your 6.1 to 7.1 and you'll be able to play the files properly.
However, it is important to note that it'll also upmix all files then - no matter if stereo, 5.1 or 6.1.
Personally, I let MC upmix everything using its JRSS algorithm, which sounds at least as good as the algorithms used by my old A/V Receiver. If thats OK for you, than that is your solution.
Some people prefer however to output the audio without upmix to an external audio processor or AVR, and in this case, you're stuck.
I keep wondering how we can solve this properly. Ideally it would automatically notice the problem and then convert 7 channel to 8 channel automatically, while not converting anything else. Or maybe there should be an option to do this, then we wouldn't have to invent auto-magic in all output plugins.
One way to get it all right now is to use the Zone Switch feature, which allows you to use separate zones with separate configuration, and you can setup the rules for 7 channels to go to a 7.1 upmix zone.
It is of course a bit of extra work to set it up and keep it maintained in case you do any changes.