Is it obligatory to repeat the ablution and prayer if, after finishing the prayer, I discover the presence of a substance on my finger that prevents water from reaching the skin, and I had known of its existence beforehand and tried to remove it, but it was not completely removed until after the prayer?

If the dye is thick, preventing water from reaching the skin, then the prayer with that ablution is invalid, and it must be removed before ablution. If it is not removed, then it is obligatory to repeat the prayer with a valid purification. However, if the dye is merely a color or a thin, fluid oil that does not prevent water from reaching the skin, then the purification is valid.