Only duty of any messenger of God is to deliver God's message.

The duty which God assigns to His messengers:

The Quran confirms that the only duty of any messenger of God is to deliver God's message. Messengers are not sent to proclaim their own personal teachings. They are called messengers of God because they deliver a message from God.

The sole duty of the messenger is the delivery (of God's message). 5:99

The same message is found in 5:92, 16:35, 16:82, 24:54, 29:18, 42:48 and 64:12.

The words in 5:99 and the similar verses are a confirmation to all believers that the sole duty of any messenger is to deliver God's message, in the case of prophet Muhammad, the message he delivered from God is the Quran.

In addition to this confirmation addressed to all believers and readers of the Quran, we also find God reminding prophet Muhammad of his sole duty:

But if they turn away, We did not send you as a guardian over them. Your sole duty (O Muhammad) is the delivery. 42:48

Following from 6:114, which confirms that God is the only lawmaker, and 5:99, which confirms that the only duty of the messenger is to deliver God's message, we have additional confirmation in Sura 10 that God alone has the authority to decree what is halal (lawful) and what is haram (prohibited):

Say, "Have you seen what God has brought down to you of provisions, of which you have made some unlawful and some lawful?" Say, "Did God give you permission, or are you fabricating lies about God?" 10:59

The significance of the words "Did God give you permission" (for what is lawful and what is unlawful) cannot be ignored. The only name mentioned in connection with granting permission for what is lawful and what is not lawful is the name of God.

We also read in Sura 66 about the incident when God reprimanded prophet Muhammad for making the error of prohibiting something that was made lawful by God, or in other words: when he played the role of the lawmaker:

O you prophet, why do you prohibit what God has made lawful for you, seeking to please your wives? God is Forgiver, Merciful. 66:1

We may wonder why are the words in 66:1 included in the Quran? Surely, God does not mean to belittle His messenger in the eyes of the believers? Not at all, this verse is deliberately included in God's Book so that whoever reads the Quran will be ascertained that the authority of lawmaking belongs to God alone.

(Quranislam)