Master program for include & not found

What causes this issue?

To be able to identify the definition location of a program module in
the source code of an ABAP/4 program include, you need an index for the
main program. Since this index depends on the main program, the system
first searches the main program containing the include. The main
program was not found for one of two possible reasons:
The include (report type 'I' in the attributes) has not yet been
incorporated in a main program.
The system index, which defines the relationship between the main
program and the include, is incorrect.
(This may be because the program has not been generated since Release
2.1D was installed.).

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

If the second case applies, you cannot correct the index by either
regenerating the main program or using Transaction SE80 (ABAP/4
Development Workbench) to regenerate the object list for the main
program. (On the initial screen of Transaction SE80, select the object
<LS>Program</> in the <LS>Object list</>, enter the main program and
press the pusbutton <LS>Generate</>.)

Error message extract from SAP system. Copyright SAP SE.