A few months ago, I did the same following these steps, you can also try this.
Upload the Size Guide Image:
Go to the Shopify Admin.
Click on ‘Files’ under ‘Settings’.
Click ‘Upload Files’ and choose the size guide image from your computer to upload it. After uploading, you’ll see a URL next to the image. Copy this URL as you’ll need it later.
Add Size Guide to Product Page:
From your Shopify Admin, go to ‘Online Store’ > ‘Themes’.
Find the theme you want to edit, click on ‘Actions’ > ‘Edit code’.
In the left side panel, search for product-template.liquid or product.liquid. The exact file can vary depending on your theme.
Decide where you want the size guide link to appear (e.g., near the size selection or the product description).
Add the following html code where you want the link to appear: