"Non credo che lui c'entri."
Non credo che lui
c'entri The last bit? entri is the 3rd person singluar present subjunctibve of entrare (enter). But what is c'
C' is the truncation of "ci", meaning "in it"; but the verb "entrarci" is more like "being able to fit", i.e. I don't believe he could physically fit in here/there.
Note that this sentence also has an idiomatic meaning: it could be intended as "I don't believe he's got anything to do with it".
Or "I don't believe he comes into it", which is both idiomatic English and a more-or-less literal translation.
Unfortunately, "I don't believe he enters IT" is not accepted. There are many problems with these later lessons. It's a shame.