Line 218 of admin/void_transaction.php throws up this error when the exist_transaction() function (also in the same file) tested in the immediate if above it returns false.

This happens when the function get_voided_entry() (defined in admin/db/voiding_db.inc) returns a valid result (1 or more rows).

This latter function extracts:

SELECT * FROM voided WHERE type=$type AND id=$type_no;

This ensures that the said entry was not voided earlier.

Therefore make sure that you have not attempted to void the same entry more than once.

Additionally, for the said error to crop up, as you are voiding a Bank Deposit entry, the function exists_bank_trans() (defined in gl/includes/db/gl_db_bank_trans.inc) must return false, that is that if there was no such entry available.

The sql executed here is:

SELECT trans_no FROM bank_trans WHERE type=$type AND trans_no=$type_no;

There will be such errors if there is a primary key violation in uniquely identifying such transactions....
Also note that the table bank_trans has NULL-able fields type and trans_no and if they have null values in some of their records, it may cause issues.
@joe: please check - should those fields be NOT NULL for best practices or are NULL values possible in FA for these fields?

The entered transaction does not exist or cannot be voided.

And the transaction could not void

Any assistance is appreciated