The so-called errors are an indication of how far behind the eightball Dreamweaver is. It shows that Dreamweaver does not have support for CSS-Grid
As a side note: have a look at the example here https://developer.mozilla.org/en-US/docs/Web/CSS/grid-template-areas for an overall layout.
Add your voice to the Bug Report here: https://dreamweaver.uservoice.com/forums/382851-adobe-dreamweaver-bugs/suggestions/33016120-using-fr...
...and do what a fair number of DW users do, turn off the Linting feature in DW and use an online validation service like: https://validator.w3.org/nu/
You can disable Linting under Edit > Preferences > Linting > uncheck the box for Enable Linting.