Your query itself doesn't make sense so I'm not quite sure what you are attempting to do. Given we don't have access to your code, computer, nor do we know anything about your project i'll try and guide you in a general direction.
First, your query is using a where clause where you are trying to get all values of rEmail that appear in a subquery. You've got a few issues here.
A subquery can only return 1 value when combined with "IN" and you are returning 2.
Your second issue is that you are looking for WHERE rEmail IN but your 2 columns you are attemping to search in are an Id column and an aggregate that have nothing to do with an email address.
With that said here is a general idea with example schema of what you need to do.
DECLARE @Reviewer TABLE (
rEmail varchar(200) NULL
);
DECLARE @PaperReviewer TABLE (
rEmail varchar(200) NULL
);
INSERT @Reviewer (rEmail) VALUES ('Test1');
INSERT @Reviewer (rEmail) VALUES ('Test2');
INSERT @PaperReviewer (rEmail) VALUES ('Test2');
SELECT * FROM @Reviewer WHERE rEmail IN (SELECT rEmail FROM @PaperReviewer);
The above example would work (providing i didn't fat finger anything, didn't run it myself) in that you should only see Test2 returned in that query.
Your alternative is to use a WHERE / EXISTS clause if you need to look up the record by multiple columns.
Given you used an IN clause i won't go down that path as i think your issue is you probably are either new to sql or don't understand your schema.
If you have questions i'll do my best to help but with the little info i have, the above should provide what you need.