Keep reading on. Paul Is talking as If he were someone still without the Spirit of God and not under the new covenant. If you continue to chapter 8 you'll see him say "there Is therefore" this Is one straight letter so context matters. I like to read 6 7 and 8 together because It gives larger context to what Paul Is saying. He isn't saying he can't stop himself from sinning after receiving the Holy Spirit.
Romans 8King James Version (KJV)
8 There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.
2 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.
3 For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:
4 That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
5 For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
6 For to be carnally minded is death; but to be spiritually minded is life and peace.
7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
8 So then they that are in the flesh cannot please God.
9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
14 For as many as are led by the Spirit of God, they are the sons of God.
Romans 6King James Version (KJV)
1 What shall we say then? Shall we continue in sin, that grace may abound?
2 God forbid. How shall we, that are dead to sin, live any longer therein?
And
6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
7 For he that is dead is freed from sin.
Also
15 What then? shall we sin, because we are not under the law, but under grace? God forbid.
16 Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?
17 But God be thanked, that ye were the servants of sin, but ye have obeyed from the heart that form of doctrine which was delivered you.
18 Being then made free from sin, ye became the servants of righteousness.
