Omnipotence is often loosely defined as, "having the ability to do anything." As some would argue God could not create a square circle, or simultaneously exist and not exist, He must not be--by definition--omnipotent.
This conclusion rests on the assumption that omnipotence stands or falls on the limits of someone's imagination; however, the ability to answer an assemblage of contradictory ideas does not determine whether God is all-powerful.
The true determination for omnipotence is, can God do whatever He intends to do, whenever and in whatever way He intends to do it?
The Bible declares He can.
- Psalm 115:3 [MEV]: "But our God is in the heavens; He does whatever He pleases."
- Daniel 4:35 [MEV]: "...He does according to His will in the army of heaven and among the inhabitants of the earth. And no one can stay His hand or say to Him, 'What have You done?'"
- Isaiah 43:13 [MEV]: "Indeed, from eternity I am He; there is no one who can deliver out of My hand; I act, and who can reverse it?"
- Luke 1:37 [MEV]: "For with God nothing will be impossible.”
- Luke 18:27 [MEV]: "He said, “What is impossible with men is possible with God.”
With regard to your question:
Hebrews 6:18 [MEV]: "So that by two immutable things, in which it was impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us."
"Impossible," as used here, has two possible meanings; either God lacks the power to lie; or God is unable to lie [Strong's G102].
As can be seen above, God can do whatever He chooses to do; whenever and however He chooses to do it. Therefore, the meaning of "impossible" in Hebrews 6:18 is that God cannot lie.
This interpretation is confirmed in Titus 1:2 [MEV]: "in hope of eternal life which God, who cannot lie, promised before the world began"
God cannot lie because it is not in His nature. As He is the God of truth [Psalm 31:5], it is impossible for Him to lie.
It is therefore true that He will not act against His nature; it is also true that He cannot act His nature.