When writing a cover letter should I write Sincerely at the end of the letter or not?

Yes

Explanation:

Make sure to offer thanks for their time and consideration, and choose a professional closing salutation such as, “Sincerely,” “Best regards” or “Thank you for your consideration.” Avoid overly familiar phrases like, “Yours,” “Cheers” or “Take care.” -How to Write an Awesome Cover Letter Closing  

yes you can use sincerely , it's acceptable

one is a red flower, and one is the sky when the sun sets.