Sed hoc primum sentio nisi in bonis amicitiam esse non posse
My translation
But, this first, I feel friendship is [not] able to exist in good; [But i dont know what to do with nisi]
Sed. hoc primum, Sentio amictiam esse non posse in bonis
Online Translation
Well, in the first place I feel that friendship can exist only among the good
If we rearrange it, it should be more clear:
Sed hoc primum sentio: amicitiam esse non posse, nisi in bonis [hominibus].
But this first I feel: that friendship can not exist, unless amongst good people.