I received a letter cp05 from the irs saying my taxes for 2018, are under review can I file and amended return while under review

NO ... do nothing while you are under review.   Until the IRS fully processes the return you will not know if they made any changes and column A of the 1040X must start with the most current version of the return. 

<a rel="nofollow" target="_blank" href="https://www.irs.gov/individuals/understanding-your-cp05-notice">https://www.irs.gov/individuals/understanding-your-cp05-notice</a>