If were are speaking of "reduction" on the purely phonetic level, it is probably not impossible that a speaker might use [ən] for on. Many kinds of simplifications are possible in fast speech.
But from a pedagogical perspective, I would never recommend that an English learner try to use /ən/ as a distinct, special "weak form" of on. I would recommend always aiming for /ɔn/ or /ɑn/ (in American English) or /ɒn/ (in British English).
Jack Windsor Lewis in "Weakform Words and Contractions for the Advanced EFL User" says that "only five prepositions [...] have stylistically distinctive weakforms": to, of, at, for, and from, and notes that
- Some students try to use weakforms of other monosyllabic prepositions than the five listed, eg of in and on, with [...] often unacceptable results. Very occasionally, in what are usually regarded as fairly markedly casual styles of speaking, mothertongue users of English do use weakforms of such words eg in Made in England /ˈmeɪd n̩ `ɪŋglənd/ I'm not in a / nɒt n̩ ə/ hurry or Cat on a hot tin roof /kat n̩ ə/ etc. These n's are by no means always syllabic if a vowel follows but they will be so if a consonant does eg when Lawrence Olivier as Lord Marchmain in the UK television Channel 4 dramatisation of Brideshead Revisited said sit in /n̩/ the open air.
Windsor Lewis's article seems to be focused on speakers who are aiming to acquire a British English accent (e.g. he mentions "/frɒm/" as a strongform of from, and "/fɒr/" as a weakform of for, neither of which is really relevant for an American English accent). For American English, I would say that n̩/ən is an acceptable weakform of the preposition in (as nloveladyallen's answer suggests), but not of on.