What the error means
Each transaction includes a recent blockhash that proves it is fresh. A blockhash is valid only for a limited time, roughly a minute. If the transaction is sent late, or the RPC node reads from a lagging view of the chain, the network cannot find the blockhash.
Common causes
- A slow, rate-limited or lagging RPC.
- Waiting too long between building and signing the transaction.
- Retrying an old signed transaction.
Fix
- Fetch a fresh blockhash and rebuild the transaction.
- Send it promptly after signing.
- Switch to a faster RPC endpoint if it keeps happening.
Background: what a Solana RPC is, and providers on the RPC providers page.