Base Apparel Coming Soon page, Vanilla HTML/CSS/JS

How best to handle the submit button and error image beside the text field? Originally I had the input in a form (class names still reflect that), but I could get a sensible layout. Then I swapped the input to being in a div, and using display flex for the input, error and submit button. The issue is that it truncates the textfield more than I'd like, particularly on the mobile size.

I tried to use position: relative and right: but couldn't get it working correctly.