What are things that must be (or must not be) mentioned in the cover letter during submitting a manuscript to the scientific journals?

The basic elements of the cover letter may be differ according to the journal policy. As a general this elements should include three important topics: a heading/salutation, the body, and the closing:

Heading and salutation:

*The heading includes the name and title of the Editor-in-Chief or

handling editor, the name of the journal, and the date.

* The salutation is a standard greeting (e.g., Dear Dr xxx:) addressed to the Editor-in-Chief or handling editor. If you cannot find the name of the appropriate editor, you can write “Dear Editor:”

*Body

The body is the heart of the cover letter; this is where you will make the case for why your paper should be granted a peer review.
Begin with a concise opening statement announcing that you are submitting a manuscript entitled [“your title”] for consideration as a Research Article, Letter, Brief Communication.
Next, provide a brief but compelling description or summary of the most important or interesting findings addressed by your manuscript.
The summary should be limited to just a few sentences. Consider the following points to help you craft your summary:
Why is your study important?
What are your most interesting findings?
What are the implications and broader significance of the findings?
What gaps in the research does your study fill?
After the description of your study, provide a brief statement of how or why the work is relevant to the scope of the target journal and of interest to its readership.

The final paragraph of the body covers a few formalities confirm that:

The research is original.
The manuscript has not been published elsewhere and is not under consideration by any other journal.
All the authors have approved of the submission of the manuscript to this journal.
There are no conflicts of interest.
Informed consent was provided (humans), and appropriate ethical standards were followed (humans and animals)—if relevant.

Suggested reviewers: Many journals invite or require authors to list recommended peer reviewers for their manuscript and to mention any individuals they would strongly prefer.

I wish this interesting for you.

Regards

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