Timeout - Stellar Developer Docs

The timeout error returns a 504 error code and occurs when either:

Horizon has not received a confirmation from the Stellar Core server that the transaction you are trying to submit to the network was included in a ledger in a timely manner, or
Horizon has not sent a response to a reverse-proxy before a specified amount of time has elapsed.

The former case may happen because there was no room for your transaction for 3 consecutive ledgers. This is because Stellar Core removes each submitted transaction from a queue. To solve this you can:

Keep resubmitting the same transaction (with the same sequence number) and wait until it finally is added to a new ledger, or
Increase the fee in order to prioritize the transaction.

Example Response for a Status Code

{
  "type": "https://stellar.org/horizon-errors/timeout",
  "title": "Timeout",
  "status": 504,
  "detail": "Your request timed out before completing.  Please try your request again. If you are submitting a transaction make sure you are sending exactly the same transaction (with the same sequence number)."
}