Your order was rejected because of a temporary technical issue at the exchange, not on Zerodha's end. You can place the order again after waiting a short while.
What causes this error
During peak trading hours, exchanges group products into partitions to handle order traffic. When a partition receives more orders than it can process, the exchange rejects incoming orders with a "slow partition" error. This is an exchange-level congestion issue that the exchange resolves automatically.
What you can do
You can place the order again after a few minutes, once the issue is resolved. The time taken can vary depending on trading volumes at the time. There is no action required on your Zerodha account.
If the issue persists, you can try placing the order on NSE instead. For example, if you are trading Crude Oil, search for Crude NCO on Kite to place the order on NSE.