As Jim said: "Assuming that you were using the Census1940 tag type". But since you are not, you will need to get a bit more clever. Because you are trying to discover what does NOT exist (no 1940 Census tag), I would suggest a two step process involving a temporary Flag.
I would first define this Flag, maybe named "Cens1940", with values 'Y' and 'N' and a default of 'N'. Now I would identify all the potential candidates using a List of People report similar to Jim's filter. Since I have a variety of tag types which might be used to set the Birth and Death dates, I would choose a filter based on the date in whatever is the "Primary" tag in the Birth and Death groups.
* Birth Group... // Year // > Is greater than // 1880 // AND
* Death Group... // Year // > Is greater than // 1940 // END
[Note the asterisk is set at the beginning of the filter for each group to select the Primary tag in that group.]
I would set the option in the Secondary Output of that LOP report to set the Flag "Cens1940" to 'Y' for these people.
Step two will be to eliminate those people from this potential set who already have a Census tag for 1940. For that I would use a List of Events report with a filter which would identify the existing 1940 Census tags. Assuming you have entered a date in these Census tags, I might use a filter on this LOE report of:
Census... // Year // Is exactly // 1940 // END
I would set the option in the Secondary Output of that LOE report to set the Flag "Cens1940" back to 'N' for "All Witnesses".
Following this two step process the Flag "Cens1940" would be 'Y' for all potential candidates who did not already have a Census tag dated in 1940. I think this is what you want? You could then get a list of these people based on this Flag.
After all this effort and the fact I am likely to want to work through this list in multiple sessions, I would probably Filter the PE for Primary Names Only based on this Flag. Then I would open the Focus Group window and remove any names there. Now I would right-click in the PE and "Add all names to Focus Group", and then un-filter the PE. I would then save this Focus Group with some appropriate group name.
Now whenever I want to work on the 1940 census, I simply load that Focus Group, right-click on the top name and change Focus to that person to review them for the census. When they are done, I would remove them from the Focus Group and re-save the group. I would probably also change their Flag to 'N'. (I use the Focus Group instead of keeping the PE filtered since performance can be affected by keeping the PE filtered as you work.)
Hope this gives you ideas,