The error UNKNOWN_FAILURE_REASON appears on Coinbase Advanced when your bot tries to place a market order with a small amount. This is a known issue with Coinbase and they are working on resolving it.
How to fix it
Switch your bot from market orders to limit orders:
Go to Configuration > Base config > Buy settings.
Change the order type to Limit (first option).
Click Save.
Contact Coinbase
If the error keeps occurring, contact Coinbase directly to report it.