You must specify an encumbrance account for the WIP Accounting Class used for maintenance (shop floor) work orders.
Navigate to the WIP Accounting Class window (Enterprise Asset Management > WIP Accounting Classes).
Define all the required fields on the page.
See Defining WIP Accounting Classes, Oracle Work in Process User's Guide).
Select the Encumbrance Accounting field, and enter the encumbrance account to be used to capture the expenses.
Save your work.