Ian Grieve reminds us that you have to have another year open to close the current year in Financial Year End Closing – Only Open Year Error Message
Financial Year End Closing – Only Open Year Error Message | azurecurve
Year-End & 1099Install, Upgrade & AdminMicrosoft & the GP Ecosystem
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.