#Problem :
after sudo apt-get update in ubuntu 18.04, started getting some erros like 
```
Err:100 http://us.archive.ubuntu.com/ubuntu bionic-backports/main arm64 Packages
  404  Not Found [IP: 91.189.91.38 80]
Ign:101 http://us.archive.ubuntu.com/ubuntu bionic-backports/universe arm64 Packages
Fetched 494 kB in 12s (41.0 kB/s)                               
Reading package lists... Done
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/main/binary-arm64/Packages  404  Not Found [IP: 91.189.88.152 80]
E: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/bionic/main/binary-arm64/Packages  404  Not Found [IP: 91.189.91.38 80]
E: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/bionic-updates/main/binary-arm64/Packages  404  Not Found [IP: 91.189.91.38 80]
E: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/bionic-backports/main/binary-arm64/Packages  404  Not Found [IP: 91.189.91.38 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

```
to fix this erros ```sources.list``` updated several time and same errors was there all the 
time.
After checking the architecture of system with command ```dpkg --print-architecture```,
terminal return ```amd64```
The real error is my system is #amd64 and those updators are looking for package in #arm64,This was casing 
issue in updating repositories with crossed architecture 

#Solution :
The soltion for this error is to remove arm64 architecture support with below command
```sudo dpkg --remove-architecture arm64```

System works fine and upto date updated and upgraded.