"Error: File /etc/sensors3.conf: Permission denied" when running htop

Snap maintainer here. That's just a warning message caused by missing permissions due to snap confinement. As a result, optional CPU temperature stats are unavailable (N/A is shown to the right of each CPU bar, if enabled).

Just updated the snap with support for manually connecting the hardware-observe interface, which fixes it (and also adds support for battery stats, btw). So, you just need to:

$ sudo snap connect htop:hardware-observe

Also, before just uninstalling the snap, note that depending on your OS release the snap might be much newer than the one in the OS repositories (eg. 3.0.4 vs 2.1.0 on Ubuntu 18.04).