I want to do the Annapurna circuit trek. What is the purpose of
Address in Nepal field in the visa application form (http://www.online.nepalimmigration.gov.np/tourist-visa)? What am I supposed to enter there if I stay in different place every night?
The Nepal Department of Immigration linked form has multiple options, to request a new visa, and extend or transfer an existing visa. If you're already in Nepal, you include your current address so that the appropriate municipality is involved. If not, indicate the first hotel/guest house in which you'll stay in both, separate fields. It's part of the Nepali government's process of granting visas on arrival to those who wish to enter the country.
The Online Tourist Visa Form works well but has strict formatting and can take a long time to get right and submit correctly without the error message.
This website (built by the Nepali International Porter Protection Group) is a guidance sheet that helps you get the formatting right to prevent the repeated error message.