3 They had not been able to celebrate it at the regular time because not enough priests had consecrated themselves and the people had not assembled in Jerusalem.

Read 2 Chronicles 30:3 Using Other Translations

For they could not keep it at that time, because the priests had not sanctified themselves sufficiently, neither had the people gathered themselves together to Jerusalem.
for they could not keep it at that time because the priests had not consecrated themselves in sufficient number, nor had the people assembled in Jerusalem--
They were unable to celebrate it at the prescribed time because not enough priests could be purified by then, and the people had not yet assembled at Jerusalem.