How to fix a config error on a custom theme website?

I have an odd error that popped up over the last 5 days. I have looked at the header liquid file but I am unable to see where this config would come from. See attached screen shot.

It shows the below on the top of the page.

gtag(‘config’, ‘UA-252375793-1’);

Website is goincase.eu if you want to look. We run a custom theme which maybe is the issue.

Can anyone help me here?