-  The knowledge

Knowledge of a regional language is necessary for a bank officer because they are required to understand what their customers are saying.

Now, let's break down the sentence and identify the errors:

"Knowledge of regional language is necessary for bank officer" - The error in this part is the missing article "a" before "bank officer." "Bank officer" refers to a specific person, and therefore, it requires an article.

"because they are to understand" - The error in this part is the use of "to" after "are." The correct preposition is "required to" instead of "to."

"what their customer says" - This part of the sentence is correct grammatically and contextually.

Now, let's understand the grammar rules related to the errors in the sentence:

Articles: Articles are used before nouns to indicate whether the noun is specific or general. "A" is used before a noun that refers to any member of a group or class, while "the" is used before a noun that refers to a specific member of a group or class. In the given sentence, "bank officer" refers to a specific person, and therefore, it requires an article "a."

Prepositions: A preposition is a word that shows the relationship between a noun or pronoun and other words in a sentence. "Required to" is the correct preposition to use when referring to something that is mandatory or essential, while "to" is used to indicate an action or direction. In the given sentence, "required to" should be used instead of "to" as the bank officer is obligated to understand the customer's language.

In conclusion, the correct sentence with no errors is:

Knowledge of a regional language is necessary for a bank officer because they are required to understand what their customers are saying.