When creating a new sitemap do I need to leave my browser open and running when I am away from the computer until the sitemap is complete?
Can I pause it and re-start it from where it was paused?
If so, can you explain how to do it without losing any data?
I have a large site and I know it will take some time to complete the crawl so I need to know. I also need to know how to re-crawl certain pages after an error is found and then fixed so that it may be included into the sitemap properly before submitting.

Yes, you will need to keep the browser open.
I believe there is a post on this site on pausing a sitemap generation, and then restarting it later.
In a nutshell, you’ll click on the stop button when you are ready to take a break / leave, etc. Then, select ‘Save Project’ from the ‘project’ tab.
do your thing.
Come back, choose ‘Open Project’ from the ‘Project’ tab, select your project, then start it again (green arrow).
Regards,
Jim.