If compiling a form in Oracle Apps 11i the below error is occured

Error

Forms 6.0 (Form Compiler) Version 6.0.8.27.0 (Production)
Forms 6.0 (Form Compiler): Release  - Production
(c) Copyright 1999 Oracle Corporation.  All rights reserved.
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bit Production
     With the Partitioning, Oracle Label Security, OLAP and Data Mining options
PL/SQL Version 8.0.6.3.0 (Production)
Oracle Procedure Builder V6.0.8.21.0 Build #0 - Production
Oracle Virtual Graphics System Version 6.0.5.39.0 (Production)
Oracle Multimedia Version 6.0.8.25.0 (Production)
Oracle Tools Integration Version 6.0.8.17.0 (Production)
Oracle Tools Common Area Version 6.0.5.32.0
Oracle CORE Version 4.0.6.0.0 - Production
PDE-PEP006 Encoded Program unit has an unknown format.

The Cause and solution to this problem is ad below.

Cause

If the 6i form has been opened in higher form builder version(10g or 11g) and saved even if no changes are done. But the form header will contain a slightly different value so when compiling the with formbuilder 6 version it will throw error "PDE-PEP006 Encoded Program unit has an unknown format".
There is not backward compatibility in Oracle forms.

Solution

Try to open the 6i backup fmb in form builder 6 and save . Then try to compile the form.

REFERENCE :- PDE-PEP006 When Opening FMB, MMB or PLL in Forms Builder? (Doc ID 294958.1)