Sprout SEO and Single channels
The error "Variable "entry" does not exist means that your current page does not have a variable called entry available. This probably means the problem lies in how you have your Sections or routing set up.

If you remove Sprout SEO from this page and just try to access entry.title in your template, you will probably receive the same error. So, it will be best to troubleshoot that and confirm you have the entry variable available to the page before troubleshooting what might be going on in Sprout SEO.

Once you know the Single is setup correctly and able to display data from the entry variable, the next question would become why is the entry variable not available to the particular part of your page where you are trying to hand off information to Sprout SEO. To answer that, it would be helpful if you posted more information on what your template looks like, where outputting the entry variable to the page directly works, and where trying to pass the entry.id variable on the page throws an error.