You need to define the viewport meta tag. Click the html settings in codepen to add it.
Your header is taking up some height in the page. This prevents the welcome from being the full available viewport height.
Either get rid of the header (by putting navbar inside the welcome section) or define it in such a way that its height appears to be zero.
There are many portfolios implemented on fcc, look thru these for ideas.
Looks like you are off by exactly 2 pixels. These two pixels are from the border you applied around everything in your css… try specifying the border around specific things and using box-sizing to include the border.