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.