The period end process checks for assets that require depreciation based on the current period or working period if enabled. It is possible to bypass the message, but once a period end has been run it cannot be reversed so before doing so please ensure that there is no outstanding depreciation for any assets based on either the current period, or current working period.
Please note that once a period has been closed, Depreciation cannot be run for that period.
To resolve this:
Click Assets
Click Options and then Asset Options
Untick the Force Depreciation before Period or Year End check box.
Re-run the period end.