Learn Portuguese (Portugal): Não deves culpar o gerente.

You can say Quem não deves culpar é o gerente (Who you must not blame is the manager). This puts the focus squarely on the manager as the specific person you shouldn't be blaming.

Why use deves here instead of tens de for 'must'?

While both can translate to 'must', dever is used for strong advice, moral obligation, or what one should do. Ter de implies a strict, unavoidable necessity or external rule. Here, telling someone they 'must not blame' the manager is strong advice, making deves the best fit.

Do I need a preposition between deves and culpar?

No. As a modal verb, dever connects directly to the following infinitive without any preposition. It is always deves culpar, never deves a culpar or deves de culpar.

Is a gerente different from a diretor?

Yes. A gerente is a day-to-day manager, usually of a specific branch, store, bank, or restaurant. A diretor is higher up the corporate ladder, like a company director or executive.