Your seeing a validation warning. You can either add a default value to your variable that you know exists (and then SSIS will validate against it and have it's required metadata) or you can set the 'ValidateExternalMetadata' property to False on your source/target components that are dynamic.

I always start with a default value for variables that are to be set dynamically at least during the development phase of the package. It helps to get the data flows all working against one source/target and then introduce the dynamic logic after your package flow works as desired. It helps sometimes with the confusion and complexity that dynamic connections add during development.