Pleroo in Greek means preach. So in Matthew 5:17 yahushuah hamashiach came to preach the law.
We know that because of the strong's exhaustive concordance and because of the verses following Matthew 5:17 that says that the law will not change until .

17 Do not think that I have come to abolish the law or the Prophets. I have not come to abolish, but to preach them. 18 For truly I say to you, until heaven and earth shall pass away, not even one iota, nor one stroke of a letter, shall pass away from the law, until everything should happen.

19 Whoever then shall break one of the least of these commandments and shall teach others the same, he will be called least in the kingdom of the heavens; but whoever shall keep and shall teach them, he will be called great in the kingdom of the heavens. 20 For I say to you that unless your righteousness shall abound above that of the scribes and Pharisees, you shall never enter into the kingdom of the heavens.

Pleroo in the Bible is translated to preach in the other places in the Bible, but Bible translators wrote fulfill here. Why?

fully preach Strong Greek #4137 - πληρόω pléroó https://www.bibliatodo.com/en/....strong-concordance/g

https://www.119ministries.com/....teachings/video-teac