If you are using the auto ad code, do not put it in a block. Blocks don’t render in the header. You add the code to your site theme’s global template in the head section.
Once it’s added to the theme, you need to make sure the code is showing when you view source. If not, it’s most likely cached to the old version still. If the code is actually there but not showing ads it’s a Google issue.
If you are NOT using Google Auto Ads and are using manual ad blocks, just use the built in IPS advertising section of the ACP. (This only works with manual ads not auto ads.)