You can add the code directly to your site header - use your ftp program to go to your Themes folder, then the folder for the theme your site is using, and download snippet.header.php. Then follow the directions for "adding the google tag directly to your webpage" in your Google Analytics panel (I'll try copy/pasting them so you don't have to track them down - except Hub posts don't like pasting code, so I changed the pointy brackets to square ones so they would show up.)

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Sign in to your Google Analytics account.
Click Admin.
At the top of the Property column, select your property.
In the Property column, click Data streams > Web.
Click the data stream for your website.
Under Google tag, click View tag instructions.
On the Installation instructions page, select Install manually:
On the screen, you’ll see the JavaScript snippet for your account's Google tag. Your Google tag is the entire section of code that appears, beginning with:

[!-- Google tag (gtag.js) --]

and ending with

[/script]

Paste your Google tag immediately after the [head] on each page of your website.

Data collection may take up to 30 minutes to begin. You can then use the Realtime report to verify that you're receiving data.

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So basically paste your google tag script after [head] (but with pointy brackets instead of square ones) at the top of your snippet.header.php file and reupload it to the same place.

If that doesn't work, or you need help finding anything, let us know 🙌