Such confusion when the Bible is so clear:
Rev 20:4 (AKJV/PCE)
(20:4) And I saw thrones, and they sat upon them, and judgment was given unto them: and [I saw] the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received [his] mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
It's not a coin, or a credit card, etc. etc. etc.
It's a physical mark. Probably along the lines of a tattoo.
The word used for MARK is:
G5480 χάραγμα charagma (cha'-rag-ma) n.
a scratch or etching, i.e. stamp (as a badge of servitude), or scupltured figure (statue).
[from charasso “to sharpen to a point” (akin to G1125 through the idea of scratching)]
KJV: graven, mark
Compare: G5482
See also: G1125
Besides the book of Revelation, there is only ONE other verse in the Bible where this word is used:
Acts 17:29 (AKJV/PCE)
(17:29) Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device.
It's a physical mark.