I tied my talit at night, and a friend mentioned to me that it is not customary to tie tzitzit at night. I looked quite extensively but did not find this rule in anywhere. Is there a rule like this, and if so, is it maakev?

Your question is addressed by many poskim, quoted comprehensively in Yabia Omer (Helek 8: Orech Hayim, Siman 3) which I have attached herein. In short, the concern is that one might think that if the tzitzit are tied at night, a time when the mitzvah of tzitzit is not applicable, then they become kosher only the following day at sunrise and they thus became kosher spontaneously and not by an act of tying (i.e., “ta’aseh v’lo min ha’asuy”). This is is roundly rejected – the most obvious example being the making of a sukkah, which also must be made in the regular way (i.e., min ha’asuy), yet one is allowed to build it long before the mitzvah is obligatory during the hag – certainly there is no difference between building the sukkah before sukkot and tying tzitzit before the light of day (Shu”t Mahagrash – in Yabia Omer, seif 3; also others in Yabia Omer, seif 5). In Seif 4, Yabia Omer brings the mahloket as to whether tzitzit is hovat gavra or hovat mana. For those that hold it is hovat gavra, tying tzitzit is not a mitzvah per se but rather a “machshir mitzvah” for when the talit will actually be worn, and thus it can be done at any time. However, for those that hold that tzitzit is a hovat mana, the mitzvah is to tie them on to the talit and there could be a concern about when to tie them. As such they are strict and require that l’chathila one tie in the day. Nevertheless, many reject this stricture, and indeed the vast majority of halachic opinion sees absolutely no reason to be strict to not tie them at night.

– Mois Navon