Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,

Under the old covenant's Levitical priesthood, there was no provision for sins of presumption, or sinning willfully.

In the verses below we can read that no sacrifice was provided for intentional sin.

Numbers 15:28 And the priest shall make an atonement for the soul that sinneth ignorantly, when he sinneth by ignorance before the LORD, to make an atonement for him; and it shall be forgiven him.

15:30 But the soul that doeth ought presumptuously, whether he be born in the land, or a stranger, the same reproacheth the LORD; and that soul shall be cut off from among his people.

15:31 Because he hath despised the word of the LORD, and hath broken his commandment, that soul shall utterly be cut off; his iniquity shall be upon him.

The book of Hebews gives Jews, undergoing persecution, the knowledge of the full scope of the new covenant that Jesus provided by His blood sacrifice.

The book of Hebrews presents Jesus as high priest after the order of Melchisedec, not after the Levitical priesthood.

Hebrews 6:20 Whither the forerunner is for us entered, even Jesus, made an high priest for ever after the order of Melchisedec.

The law, as well as the priesthood, was changed.

Hebrews 7:12 For the priesthood being changed, there is made of necessity a change also of the law.

Under the new priesthood, and change in the law, there still remained no sacrifice for willful sin.

Below is a warning to the Hebrews about reverting back to the Levitical priesthood, and it's sacrifices. They are warned about willfully rejecting the blood sacrifice of Christ as being sufficent to remove all their sin, and bring in the new covenant.

Hebrews 10:29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?