I checked that I had insurance on the form thinking I was covered, but I wasn't & have filed. been rejected. how can I fix this & refile ?

If you answered a question incorrectly and the return was rejected, you can go back in to the return, correct the answer and re-file.   The only question about health insurance is the one about a 1095A for health insurance if you had marketplace health insurance ("Obamacare" from healthcare.gov)   If you did not have that kind of insurance you say NO to the question about it and move on.

You received an email that we cannot see that tells you why your tax return was rejected.  

Some rejected returns can be fixed and re-filed.  Some have “un-fixable” reasons for the rejection and have to be mailed instead. What is the rejection code or rejection message? 

 If it was rejected for AGI—-be aware that many 2020 returns took so long for the IRS to process that your 2020 AGI might not be in the IRS system.   Try using zero instead.

If filing with zero is rejected then file again but select the option that you did not file last year.  The IRS does not see that question and it will allow you to e-file with no AGI question.

https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/00/26311

If that does not work and you still cannot e-file, then print, sign, and mail your tax return.