The United Bibles Societies Fifth edition of the Greek New Testament (UBS5) does not allow a root search from the context Menu. Roots are available for the UBS fourth edition (UBS4), as well as for the Nestle-Aland 28th edition and SBL Greek New Testament. The root tags are there for reverse interlinear English Bibles too.
It appears that the UBS5 in Logos does not have words tagged by root. Why is this missing? The Greek text is the same as Nestle-Aland 28th edition, except for a few punctuation marks, so it should be easy enough to include.
This is disappointing since UBS5 is my preferred Greek New Testament for day to day use. It just shows how rarely I use root searches that I did not know it was missing.