Shirk means associating partners with Allah (S.W.T). If you associate partners with Allah (S.W.T), you're repentance from Allah (S.W.T) is not accepted because it is the worst sin in Islam. The only time it is accepted is before you die. It is stated in the Qur'an: "Allah (S.W.T) forgives not that partners should be set up with him, but he forgives anything else, to whom he pleases, to set up partners with Allah (S.W.T) is to devise a sin most heinous indeed" (Quran Al-Nisa 4:48). Even in the Shahadah, it says "La illah ha illal lah, muhammad ur rasoolullah. There is no god but Allah and Muhammad is his slave/ messenger.
