The ChannelMix Predictive Model tests thousands of scenarios to find the best ways to distribute a budget between all your channels (platforms, etc.). If we are unable to find any scenarios that meet or exceed the requested goal, we deem in unattainable. That simply means that the model sees no way to achieve your goal.
What should I do if I still think it's a reasonable goal?
A reasonable goal can be classified unattainable by the ChannelMix Predictive Model if it has incomplete information about your market. If you have a goal you think is reasonable, think about the factors beyond marketing spend that could contribute to achieving your goal and talk to your CSM about ways ChannelMix can integrate that information into the model. The more information the model has, the better it will perform leading to a better understanding of what goals truly are attainable and unattainable.