It means basically what it says: according to the "balance test", the tables are out of balance. But the balance algorithm cannot figure out how to move players in such a way as to balance the tables.

The "balance test" is very straightforward. It simply takes the number of players seated at the table with the most players and subtracts the number of players seated at the table with the fewest players. If this difference is greater than the "player disparity" setting (configured on the Tables tab, but defaults to "1"), then the software thinks the tables are out of balance.

Since Table 16 and Table 12 both have 10 players seated, and Table 17 only has 8, there's a difference of 2 players between Tables 16 and Table 17, as well as between Table 12 and Table 17. So the player disparity is 2, which is greater than the configured maximum disparity of 1. This triggers the flag that tables are not balanced. However, with the two empty seats on Table 17 marked as Unavailable, the balancing algorithm cannot move players such that the "balance" test is satisfied. So the software is essentially throwing its hands up and saying "I can't figure out what to do to make this balanced."

Make one of those unavailable seats available and the software will figure things out...