What causes this issue?

In the function module, an attempt was made to access a field of the
structure. The function module does not support access to the structure
(or access to the requested field of the structure).

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?

Determine which expression causes this error. To do so:
Call transaction BRF.
In the toolbar of the navigation tree, choose the arrow on the right,
next to |@13@ with the quick info <ZK>Find BRF Object</> and then
<ZK>Find Expression</>.
The system displays the dialog screen <ZK>Find BRF Expression</>.
Call the input help for the <ZK>Expression</> field.
On the next dialog screen, choose one of the following tab pages:
<ZK>Field of a Structure</>
<ZK>Field of a Row of an Internal Table</>
Check the expressions of the structure.
To do so:
Choose an expression by double-clicking.
<ZH>Note</>: If the system opens the expression in display mode, switch
to change mode.
In the <ZK>Administrative Data</> section, choose |@38@ with the quick
info <ZK>Check</>.
Analyze the messages that are displayed.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.