John Gill Commentary Ezra 5:16

"Then came the same Sheshbazzar, and laid the foundations of the house of God which is in Jerusalem: and since that time even until now hath it been in building, and yet it is not completed." — Ezra 5:16 (ASV)

Then came the same Sheshbazzar, and laid the foundation of the
house of God which is in Jerusalem
Which makes it clear, that by Sheshbazzar is meant Zerubbabel; for he it was that laid the foundation of the temple, or at least by whose order it was laid, see (Zechariah 4:9)

and since that time even until now ;
from the first of Cyrus to the second of Darius, a space of about eighteen years, and just seventy from the destruction of the temple:

has it been in building, and yet it is not finished;
the work going on slowly, not without interruption and intermission, through the enmity of the Samaritans to them, who had made false representations of them; but these men, Tatnai and those with him, as the Jews gave them a very particular account of things, as above, so they fairly and fully related them in this their letter to the king.