Jesus' words in Jn 6:63 have essentially a two-fold meaning: it is only the Spirit that can accomplish the miracle of the Eucharist, and it is only the Spirit that can empower us to believe in the miracle. If you are relying on the power of the flesh, you aren’t going to see it!
You must look at the whole chapter of John 6.
Jews sought Jesus to get the food like Manna, but Jesus concluded that they should believe in Him for their daily lives. Believing in Jesus would have got the Jews the daily necessities as the Spirit of Jesus could have led their lives.
Read John 6
25 And when they had found him on the other side of the sea, they said unto him, Rabbi, when camest thou hither?
26 Jesus answered them and said, Verily, verily, I say unto you, Ye seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled.
27 Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
28 Then said they unto him, What shall we do, that we might work the works of God?
29 Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.
30 They said therefore unto him, What sign shewest thou then, that we may see, and believe thee? what dost thou work?
31 Our fathers did eat manna in the desert; as it is written, He gave them bread from heaven to eat.
32 Then Jesus said unto them, Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven.
33 For the bread of God is he which cometh down from heaven, and giveth life unto the world.
34 Then said they unto him, Lord, evermore give us this bread.
35 And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.
36 But I said unto you, That ye also have seen me, and believe not.
These verses don't say about the Eucharist but teach us that Jesus is our daily Bread as He would live within us by Spirit, and the Spirit would lead us in all matters of our lives. Jesus is better than Manna as He is the Eternal Life.
This is not talking about the ritual with drinking blood and eating the roasted flesh, but talking about Being Born Again by Spirit, then the Spirit will lead our lives in the best ways.
Roman Catholics misunderstand or don't understand these verses because they are not Born Again in Lord Jesus Christ, sadly.
Eliyahu