Go to My Site ->Site ->Pages and edit your home page.
Click on the logo image there – it looks like you’ve made some HTML changes to that block that broke it, so you’ll have to fix that first: Click the three dots top-right of the cover image, and select “Attempt Recovery”.
That should set the blocks back to default, and now you should see the signup button, along with a text box where you can add the link the button should go to.
That button is meant to be linked to something, typically a newsletter signup form or something similar, so you’ll first need to set that up before you’ll be able to link to it.
Also note that if you just want a button so people can follow your blog posts, you don’t need that Signup button – your site automatically shows a Follow button bottom-right to all visitors. That button will appear as soon as you have two or more blog posts published on the site.