1. Open projects that show a difference between columns for actual this period and actual costs or units. There is stored period data in any projects that show a difference between those fields.
2. In Activity view, select View, Show on top, Activity Usage Spreadsheet then View, Timescale and set to Financial Period.
3. Right-click the Activity Usage Spreadsheet and select Spreadsheet Fields and select the appropriate Financial Period Value field(s).
4. Select view, Collapse All to find project with financial period data during the period you want to delete.
5. Either delete the project or remove the project's financial period data.
6. Once all projects show no financial period data in the period you want to delete, you should be able to delete the financial period without issue.