Sync? controls whether the background job keeps pulling new transactions for that account. Click it and confirm Are you sure? to turn syncing on or off; balances continue to display either way. The toggle is disabled (greyed) when the account cannot sync yet: either the account has never completed its initial import (an Import button is shown on the institution card) or the bank requires you to log in again (a Re-authenticate button is shown). Fix that first and the toggle becomes available.