August 28th, 2021 | Updated on March 9th, 2022
Only Unspent Transaction Outputs, or UTXOs, can be used to be spent as an input in another transaction whereas spent outputs are already spent hence can’t be spent again. (Difficult to grasp? Stay with me.) You always need a UTXO or an unspent transaction output to make a transaction.
If you don’t have an unspent transaction output, it simply means you don’t have any Bitcoin. This mainly happens due to the protocol rules which Satoshi Nakamoto had defined in Bitcoin to prevent double spending.