49 נַסּוֹת (nassot) is the Piel infinitive construct; it forms the purpose of God’s coming with all the accompanying phenomena. The verb can mean “to try, test, prove.” The sense of “prove” fits this context best because the terrifying phenomena were intended to put the fear of God in their hearts so that they would obey. In other words, God was inspiring them to obey, not simply testing to see if they would.
50 The suffix on the noun is an objective genitive, referring to the fear that the people would have of God ( §135.m).
51 The negative form לְבִלְתִּי (lévilti) is used here with the imperfect tense (see for other examples §152.x). This gives the imperfect the nuance of a final imperfect: that you might not sin. Others: to keep you from sin.

In other versions

Moses said to the people, “Do not fear, for God has come to test you, that the fear of him may be before you, that you may not sin.”

Moses said to the people, “Do not be afraid. God has come to test you, so that the fear of God will be with you to keep you from sinning.”

Moses responded to the people, “Don’t be afraid, for God has come to test you, so that you will fear him and will not sin.”

“Don’t be afraid,” Moses answered them, “for God has come in this way to test you, and so that your fear of him will keep you from sinning!”

And Moses said to the people, “Do not be afraid. God has come to test you so that his fear will be before you so that you do not sin.”

And Moses said unto the people, Fear not: for God is come to prove you, and that his fear may be before your faces, that ye sin not.

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