The best way to ensure your cover letter is error-free is to proofread it carefully. Make sure to check for spelling and grammar mistakes, as well as typos. You may want to have someone else read it over as well to catch any errors you may have missed.
It can also be helpful to use a grammar checking tool like Grammarly to help you identify and fix any mistakes. Another tip is to read your cover letter out loud to yourself to catch any errors or awkward phrasing.
Take your time proofreading your cover letter and make sure it is perfect before sending it off. A well-written and error-free cover letter can make a great impression on potential employers and help you get one step closer to landing your dream job.