If you hover over the reasons, it lists the names of the players:
Position familiarity is a rating from 1-20
1 - 4 Ineffectual
5 - 8 Awkward
9 - 11 Unconvincing
12 - 14 Competent
15 - 17 Accomplished
18 - 20 Natural
I've just tested and it appears that to be tutored, both players must be at least 18 in a shared position.
Shilow Tracey is 20 at right back.
At the moment he cannot be tutored by anyone at the club. If I hover over the reasons why, I can see the names of the seven players that do not share a position.
Let's take one at random, Fernando Llorente. He has a rating of 1 for right back.
Changing his familiarity at right back to 17 (anywhere between 15 - 17 is accomplished. So this means Tracey is 20, natural, Llorente is 17, accomplished) makes no difference. He still cannot tutor Tracey.
Changing his familiarity to 18, natural and...
Changing Tracey to 17, accomplished at right back stops it working. Changing him to 18 so both him and Llorente are 18, natural and it works again.