There are two aspects to consider when answering this question. Firstly, this soliloquy begins by pointing out that the sin of Claudius is actually pretty bad as far as sins go, because it is repeats the very first act of murder that we are told about in the Bible, when Cain killed his brother Abel. Therefore, according to Claudius, it is especially bad:
Oh, my offence is rank, it smells to Heaven,
It hat the primal eldest curse upon't.
A brother's murder.
Secondly, Claudius says that he has no chance of being forgiven when he still has everything he gained from committing that murder.
Forgive me my foul murder:
That cannot be, since I am still possess'd
Of those effects for which I did the murther,
My crown, mine own ambition, and my Queen.
Claudius says that in this world you can often buy your way out of justice, keeping what you gain by committing crimes and avoiding punishment, but then in heaven, no such option is available, because all of our faults are evident and cannot be hidden. Forgiveness or absolution cannot be received whilst he still retains the crown, Gertrude, and his own ambition.