Learn Turkish: Yönetim, tecrübesi olmadığından dolayı hata yaptı. - Management made a mistake because they lacked experience.

This uses the causal clause structure you are learning. The root ol is used for "to have" or "to be". We add the negative -ma, the participle -dık (which mutates to -dığ), the 3rd person possessive -ı (referring to the management), the buffer -n-, and the ablative suffix -dan. With the postposition dolayı (due to), it literally means "due to its not having".

Is the word dolayı required here?

No, it is completely optional. Saying Yönetim, tecrübesi olmadığından hata yaptı means exactly the same thing. The ablative suffix -dan attached to the -dığı form already means "because of" or "since". Adding dolayı just adds a little extra emphasis to the cause.

Why does the word for experience have a suffix here (tecrübesi)?

To say someone "has" or "lacks" something in this kind of clause, the item possessed needs a possessive suffix. Tecrübe (experience) takes the 3rd person singular possessive -(s)i to become tecrübesi (its experience), linking it to the 3rd person subject, Yönetim (management).

The English translation says "they made a mistake", but the Turkish verb yaptı is singular. Why?