When an auditor's report contains an expression that an opinion cannot ...

When an auditor's report contains an expression that an opinion cannot be expressed, the reason is to prevent misunderstanding of the degree of responsibility assumed when the auditor's name is associated with financial statements. 

Moreover, if an auditor is unable to obtain sufficient appropriate audit evidence to support management's assertions about the nature of a matter (i.e. presentation/disclosure), consideration must be made to express a qualified opinion or to disclaim an opinion because of a scope limitation.

Quick related but not so related note, working papers document audit work performed, including evidence of supervision and review.

​cc: Audit