Is it obligatory to repeat the ghusl and the prayer, and is the fast invalidated, if someone in a state of janabah performs ghusl while having a thick cream on them that is not removed by water?

If what was placed on the skin prevents water from reaching it, then it must be removed before performing ghusl (ritual bath); otherwise, the ghusl is invalid. Only the spot that water did not reach needs to be washed, because continuous succession (muwalat) is not a condition for ghusl. The prayers that were performed with an invalid ghusl must be repeated. As for fasting, it is valid because ghusl from janabah (major ritual impurity) is not a condition for its validity.