As asked in title. Thanks for your time!
Welcome to Stack Exchange, CiviMail will create a plain text version of your email for those using a plain text client. So there is no need to enter content in the plain test area unless you wish folks using plain text to have a specially formatted message or you just want to send your email in plain text that is when you would use the plain text field.
If you have already entered text, delete it and it will be re-generated.
Here is a link to more info: https://docs.civicrm.org/user/en/latest/email/mass-mailings-using-civimail/#mailing-tab
Hope this helps