Geth RPC docs:
eth_getTransactionReceipt
Returns the receipt of a transaction by transaction hash.
Note That the receipt is not available for pending transactions.
What is the reason why Receipt not available for pending transactions?
A Transaction receipt gives data about the state after the transaction is mined, the hash of the trie which stores these receipts is included in the block headers. If you want to check the details of a pending transaction, then you can use eth_getTransaction
method.