Hi @DaisyPJ,
so I believe there are some changes you still have to do.
I tried it on my tenant and it seems to be working.
You still need the sign in but want to delete the register tab as far as I understand.
First create the content snippet as explained here, as you will have to modify the SignIn Page with JavaScript.
https://ulrikke.akerbak.com/2020/02/16/customize-the-sign-in-and-registration-page-in-power-app-portals/
You later on need to replace the content with:
<script>
$("#content-container").addClass("signinpage");
let register = $('#content > ul > li:nth-child(2)')
register.remove()
</script>
if the selector is different on your page. Use the google chrome selector and paste it inside $('insert here')
After that your page should look like that:

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Hope that helps. If it is the solution please give kudos and mark it as such.