請問呢封投訴信有咩錯呀.?唔該幫幫手呀..萬分感謝

It is certanly our goal [aim] to give our customers the best service we can. [not ,]

But at times [not "but sometimes"] things [not thing] go wrong and out side our control. [shorten the sentence with more direct meaning, deleting "so we are not able to do our aim"].

The night you and your wife came to the restaurant our [not the] chief waiter was on sick leave. [not ill,]

[no need ",and unfortunate his job was taken up by a new waiter".]

His substitute is inexperienced and thus made all mistakes you listed on your letter. [rewrote "He very nervously and made the many mistakes you listed in your letter."]

Adding [To add] to the chaos that night there was [had] an interruption of [to the] [c]electricity supply at [to] the kitchen, which made delivery of dishes went into wrong order [don't understand your meaning so I just guessed: the live of the dishes cooked in the wrong order. ]

It is our aim to give the best service to our customers. However, we cannot able to control it when things go wrong accidently.
At that night you and your wife came to the restaurant, the chief waiter was ill, therefore his job was replaced by a new waiter at that time. The new waiter was very nervous, thus he made mistakes as you mentioned in your letter.
There was chaos in the kitchen during the interruption to the electricity supply, which caused the dishes cooked in the wrong order.