"Negative values are invalid for a pie chart" on one of the dashboard reports means that an Item or Mod with a negative value (eg: -$0.50) has been set up and has been put into a category that totals up to a negative number. This category is usually "Other".
If you create a mod called Keep Cup for -$0.50 and assign it to the category "Other", but nothing else is assigned to the category, at the end of the day the total for the category of 10 keep cups will be -$5.
A negative amount can't be displayed on a pie chart.
For the chart to work properly, the Keep Cup mod could have been categorised as "Coffee" as it relates to that category.
How to Fix
Locate the mod or item with the negative value (if you're stuck, check out the Items Moved report on the Stock page and use the category picker near the top to go through each category - and find the one with a total negative value.)
Then, head to the Menu page, locate the item or mod and edit its category to one that it relates to and Save. This should resolve the issue.