Welcome to the Coinbase Developer Forum, @apaulo! We understand that you encountered an error when trying to place a Market Order. Upon checking the Body params you provided, it looks like you were using the base_size parameter when trying to place a BUY order. Please note that the base_size parameter is applicable for SELL orders. Hence, the Market buys must parameterize in quote currency error.
For BUY Orders, quote_size is a required parameter in order for you to place a Market - BUY order. With that being said, we suggest removing the base_size parameter and replacing it with quote_size in order for you to proceed with the Market - Buy Orders.
The Body Params of your request should look like this: