不得其門而入 [－－－门－－]

Jyutping bat1 dak1 kei4 mun4 ji4 jap6
Pinyin bù dé qí mén ér rù

Definitions (CC-CEDICT)
  1. to be unable to get into (a house, an organization, a field of study, a particular type of career etc)
Definitions (Wiktionary)
  1. phrase, idiomatic
    to not have an "in"; to be unable to crack into (a market); to try to find an in without any success; to be unable to gain entrance