Here is a good article that I think would help in deleting your manual account. Essentially, you will need to delete the rows associated with the manual account from the balance history tab. The article below goes into detail on how to do this.
In that case, I would go to your Balance history sheet, filter on the account, and delete all rows.If you created any overrides on the Accounts sheet, you may also want to delete those entries as well.
I think I need some help with where the balance history is? I see my balance tab, but when I delete it there, it doesn’t delete everywhere else. Any thoughts?