My Reef pi comes up with this error all the time.
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I have tried several times. I am having problems with version 6.0 of Reef Pi. Thinking of rolling back to version 5.3.Means he wasn't able to find a specific temperature sensor, which can happen due to several things, I guess.
Software issues during communication, Hardware issues of incorrect timings and or interference in the wron moment.
Have you tried turning it off and on again ? ^^
[Amend]: Restarting The PI in this case.