That the correct translation is "the spirit" is widely recognized; eg:

Ellicott:

(21) A spirit.—It should be the spirit. The definite article is explained by some, perhaps rather weakly, as simply anticipatory of the description which follows. Others take the phrase to signify “the spirit of prophecy,” a kind of emanation from the Godhead, looked upon as the medium of the prophetic inspiration, which is an expression conceivable, but certainly unprecedented. ...

Cambridge:

21. And there came forth a spirit The Hebrew has ‘the spirit’ as is noted on the margin of the R.V. It seems therefore to imply some definite power which imparted to prophets their gifts; the prophetic spirit. That God allowed this power to delude Ahab was because of the king’s persistence in evil. God therefore gives him over to it, and causes the prophets whom he has chosen for himself, to the rejection of Micaiah and such as he, to be the instruments of his destruction. Thus when Isaiah is sent to rebellious Israel (Isaiah 6:10) his mission is described as of this nature. God says to him ‘Make the heart of this people fat and make their ears heavy, and shut their eyes, lest they see with their eyes, and hear with their ears, and understand with their hearts.’ In this wise and with like effect comes the spirit from God into the mouths of Ahab’s four hundred.

Pulpit:

Verse 21. - And there came forth a spirit [Heb. the spirit. By some, especially of the earlier commentators, understood of the evil spirit. But the view now generally adopted (Thenius, Keil, Bahr) is that "the spirit of prophecy" is meant, "the power which, going forth from God and taking possession of a man, makes him a prophet (1 Samuel 10:6, 10; 1 Samuel 19:20, 23). The נָביא is the אִישׁ הָרוּחַ (Hosea 9:7)" Bahr. This power is here personified], and stood before the Lord, and said, I [emphatic in the Hebrew] will persuade [or entice] him. 1 Kings 22:21

Who was "the spirit"? As can be seen from the above, opinion is divided. However, we should not forget Isa 45:7 -

I form the light and create the darkness; I bring prosperity and create calamity. I, the LORD, do all these things.

As the Cambridge commentary observes, "the spirit" may have been the Spirit of God who allowed the deceiving to occur in a receptive heart. However, this is outside the scope of this question and should be the subject of a separate query.

That the correct translation of 1 Kings 22:21 is "the spirit" is widely recognized, eg:

Ellicott:

(21) A spirit.—It should be the spirit. The definite article is explained by some, perhaps rather weakly, as simply anticipatory of the description which follows. Others take the phrase to signify “the spirit of prophecy,” a kind of emanation from the Godhead, looked upon as the medium of the prophetic inspiration, which is an expression conceivable, but certainly unprecedented. ...

Cambridge:

21. And there came forth a spirit The Hebrew has ‘the spirit’ as is noted on the margin of the R.V. It seems therefore to imply some definite power which imparted to prophets their gifts; the prophetic spirit. That God allowed this power to delude Ahab was because of the king’s persistence in evil. God therefore gives him over to it, and causes the prophets whom he has chosen for himself, to the rejection of Micaiah and such as he, to be the instruments of his destruction. Thus when Isaiah is sent to rebellious Israel (Isaiah 6:10) his mission is described as of this nature. God says to him ‘Make the heart of this people fat and make their ears heavy, and shut their eyes, lest they see with their eyes, and hear with their ears, and understand with their hearts.’ In this wise and with like effect comes the spirit from God into the mouths of Ahab’s four hundred.

Pulpit:

Verse 21. - And there came forth a spirit [Heb. the spirit. By some, especially of the earlier commentators, understood of the evil spirit. But the view now generally adopted (Thenius, Keil, Bahr) is that "the spirit of prophecy" is meant, "the power which, going forth from God and taking possession of a man, makes him a prophet (1 Samuel 10:6, 10; 1 Samuel 19:20, 23). The נָביא is the אִישׁ הָרוּחַ (Hosea 9:7)" Bahr. This power is here personified], and stood before the Lord, and said, I [emphatic in the Hebrew] will persuade [or entice] him. 1 Kings 22:21

Who was "the spirit"? As can be seen from the above, opinion is divided. However, we should not forget Isa 45:7 -

I form the light and create the darkness; I bring prosperity and create calamity. I, the LORD, do all these things.

As the Cambridge commentary observes, "the spirit" may have been the Spirit of God who allowed the deceiving to occur in a receptive heart. However, this is outside the scope of this question and should be the subject of a separate query.