As Ubuntu bionic release does not support i386 architecture, you can fix the error by forcing only 64-bit architecture. You need to do the followings:
1. Change directory to sources.list.d.
cd /etc/apt/sources.list.d
2. Open the file nginx.list and replace
deb http: //nginx.org/packages/mainline/ubuntu/ bionic nginx
With
deb [arch=amd64] http: //nginx.org/packages/mainline/ubuntu/ bionic nginx