Can a TiVo Series 3 work with both a OTA antenna and 2 cable cards?

So I have a lifetime series 3 and want to try cutting the cord from my cable provider.

But I wanted to "stage it" and see how I liked it before formally ending my cable subscription.

But it seems the unit won't let me use "both" at the same time. I get an error message saying it won't let me proceed.

I'm getting the impression I have to remove my cable cards and only use the antenna.

Is that correct?

What am I doing wrong?