Not withstanding Richard’s comment - it looks from your screenshot as though you have pasted ALL the code into the page.
The instructions from your developer clearly say to put one line of code in the HEAD section and the rest in the BODY (which is your page).
Go to Site administration > Appearance > Additional HTML and add the line that needs to be in the HEAD section to the first box (Within HEAD). Then go to the page that you created and make sure you only include the code that your developer says needs to in BODY.
The instructions from your developer clearly say to put one line of code in the HEAD section and the rest in the BODY (which is your page).
Go to Site administration > Appearance > Additional HTML and add the line that needs to be in the HEAD section to the first box (Within HEAD). Then go to the page that you created and make sure you only include the code that your developer says needs to in BODY.