Sanballat's unsealed letter claimed Nehemiah was rebuilding the wall in order to become what?

Answer: Their king

The letter alleged a revolt, with prophets proclaiming "a king in Judah." Nehemiah replied that Sanballat was inventing it all.

“that read: “It is reported among the nations—and Geshem agrees—that you and the Jews are plotting to revolt, and this is why you are building the wall. According to these reports, you are to become their king,”— Nehemiah 6:6 (BSB)