Hi, is it permissible to put rabbeinu tam tefilin during ben hashemashot for one who puts it everyday but couldn't put it earlier in that day , as not to lose the mitsva?
Answer:
Yes. Although one should try to do this before Shekiya, if one was unable to he can do so during bein hashemashos.
Best wishes.
Sources:
See Shulchan Aruch 30:2 and Mishnah Berurah; Biur Halachah s.v. veyesh lehachria (concerning the berachah).
The Chayei Adam (14:15) actually writes that one can recite the blessing over Tefillin during Beis Hashemashos, and this is ruled by make authorities (see Artzos Chaim; Nidchei Yisrael Chap. 6; Yabia Omer 7:7), and this is the principle halachah. However, for Rabbeinu Tam Tefillin this is not relevant, since a berachah is not recited in any case.