You receive an error message in the Modern UX when trying to create a financial plan, "You need to associate investment with an entity." This message is received a soon as you click on the financials or financial plans. If you get out of the project and go back in again, you are able to create the financial plan. The error is not received in Classic.
Release : 16.0.0
Component : CLARITY FINANCIAL MANAGEMENT
This message, "You need to associate investment with an entity." is a warning and to fix check the project department field. If a department is not associated /selected, select the correct one for your organization and go back to the project financials to see if the warning is still present.