Some browsers, talking to you IE and iOS pre 9.0, do not support clamp. To make the text styled (non-responsive), you need to create a fallback so that the page will not break. Add this code in the field 18px; font-size: clamp(1rem, calc(1rem... without a trailing ';' at the end of the clamp part.
18px; font-size: clamp(1.913rem, calc( 12px + 2.475vw ), 2.587rem)
This code will make the browser read the font size in pixels and then move to the clamp statement. If the browser does not support clamp, then it will keep the pixel property.