The bank granted me a bridging loan. To date, I've been unable to sell my property. The bank tells me that this is not a legitimate reason for a non-repayment on the contractual due date. Is this true?

Yes. When your bridging loan contract expires and you are unable to repay it, either you succeed in negotiating an extension of the repayment term with your bank, or the bank may demand repayment of the entire loan amount on the agreed date.