Can't get channel6 tuning to work.Always return to 0 on refreshing

Hi, guys. I've just bought a APM 2.6 and loaded 3.2.1 firmware, quad x.

I tried to use Dave C''s PID tuning method. But encountered two problems.

first,I can't seem to set rate PIDs to zero, Is there a min limit on the PIDs? why? 

Also, I can't save the

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