When you see the error 'Preview_Insufficient_funds' in your Coinbase trading bot, it means that you are trying to place an order with a larger amount of the quote currency than you have available to trade. This error can be prevented by using a slightly smaller amount of the quote currency for your buy order.

Example of the error in the trading bot output:

05/21/2024 6:41:56 PM ERROR Error placing buy order, data: {"error":"PREVIEW_INSUFFICIENT_FUND: Insufficient balance in source account","orderNumber":""}