Unfortunately Netflix would rather hire mercenaries to hunt us peasants down than allow for the heinous criminal offense of someone taking a screenshot.
There are some guides around, like https://www.wikihow.com/Screenshot-Netflix, that _might_ work (just keep in mind Netflix has people hired fulltime just to make their service less useable, so things can get out of date fast); however, avoid _any_ guide/tutorial that suggests installing some extension or add-on
Most screencaps generally come from local downloads of episodes after some angel somewhere rips the video and disseminates it through torrents or other services.