If your company has enabled, Restrict Projects by BU and Require Projects then you must make sure every business units have at least 1 active project associated with it. By not following this step may prevent users from submitting an invoice or expense report.
Below is the message that would be seen on each of the transaction module tabs.
1. Purchasing tab.
2. Invoices tab.
3. Expense Reports tab.
To see instruction for associating Projects to business units, see Managing Project/BU Association article.