You are too late to "fix" it and e-file. Now you can only print, sign and mail your tax return. The most common reason for rejection is AGI. The email that told you it was rejected has the rejection code or rejection message (check your spam folder) AGI no longer matters since you cannot e-file anyhow and you only needed it to e-file.
Online tax preparation and e-filing for 2018 returns is now permanently closed. You had until midnight on Tuesday, October 15 to finish and attempt to e-file; if your e-file was rejected you had until October 20 to make corrections and re-file. Now it is too late to use online software or to e-file. If you have any changes/corrections at all to make to your 2018 return, you now have to complete it using desktop software, which you must download to a full PC or Mac (not to a mobile device).
If you already paid for your online software you can ask customer support for a download of the desktop software.
They are available from 5 a.m. to 5 p.m. Pacific time Monday-Friday.
If you are getting a refund, there is no penalty for filing late. If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe. Only the IRS will calculate this—TurboTax will not calculate it.