The expense management workspace currently has a limitation when the expense re-imagined feature is enabled as it prompts error" The activity number is missing. The project that is associated with this expense requires you to enter an activity number, with a postponed bug (675860). 

The workaround is to disable the feature which isn't ideal as it affects the user's ability to add cash advances to the expense lines. 

The business requirement is to fix the bug to allow the users to benefit from the cash advance functionality introduced within the Expense reports re-imagined feature.