Can pregnancy occur if precum is on vagina but no penetration and no ejaculation. just pre-ejaculatory fluid?

No. But if worried..: Pregnancy won't happen if the sperm can't get into the vagina, or if the sperm are just a little bit into the vagina and can't swim all the way up through the cervix and beyond. Life sometimes will surprise people, because people having sex are not thinking of the details (like exactly when and where a finger, a penis, or sperm are located). One can do a pregnancy test 3 weeks after sex.