The problem is solved now. The problem was that there was limitation of external domains in site collection feature. If want to limit external sharing of site, follow below steps:
1- Got to https://portal.office.com and click on Admins.
2- Click on Sharepoint in Admin Centers.
3- Click "More features" in left side.
4- Click "Open" on "Classic site collection page" in right side.
5- Select the site collection of the site and click on Sharing.
6- Go to Site Collection additional settings and add gmail.com (if there is already domain added, put space between two domains).
7- Click save button.
Now access the site using external user account.
