Old Testament readings use the Septuagint , the Scripture the apostles quoted. Masoretic numbering shown for reference.Learn why
LXX

2 Chronicles 20:21

And he took counsel with the people, and set appointed men to sing psalms and praises, to give thanks, and sing the holy songs of praise in going forth before the host: and they said, Give thanks to the Lord, for his mercy [endures] for ever.

Read in other translations