The Church has power to forgive sins and this authority is exercised by the members of her clergy to whom the Church has given the permission to do so.
However there are some sins that the Church has reserved to the Pope and bishops to forgive. Some of them are as follows:
1. Physically attacking the Pope including attempting to assassinate him.
2. Breaking the “Seal of the Confessional” by a priest, i.e. violating the confidentiality of the confession by revealing the sins and identity of a penitent to anyone.
3. A priest absolving an accomplice in sin.
4. Participating in abortion which includes advising, paying for, or carrying out the act and after seeking admission into the order of priests or deacons in the Church. When someone who has committed this crime in the past wants to be ordained, they must first receive dispensation from the Pope.
5. Defiling the Eucharist, the Body and Blood of Christ is extremely grievous and can be dealt with only by the Pope.
(from Catholic Say)