Character string not complete

What causes this issue?

The syntax of the boolean statement is incorrect.
Syntax error in boolean statement. A literal string has been declared
without the terminating '.
E.g. <tab> $fld = '4556

System Response

Code generation aborted.

How to fix this error?

Correct the literal string by terminating it with a '.
E.g. <tab> $fld = '4556'

Error message extract from SAP system. Copyright SAP SE.