apt update

 404  Not Found [IP: 202.38.95.110 443]
Ign:64 https://mirrors.ustc.edu.cn/ubuntu bionic-updates/restricted arm64 Packages                               
Ign:68 https://mirrors.ustc.edu.cn/ubuntu bionic-updates/universe arm64 Packages                                 
Ign:74 https://mirrors.ustc.edu.cn/ubuntu bionic-updates/multiverse arm64 Packages                               
Err:82 https://mirrors.ustc.edu.cn/ubuntu bionic-backports/main arm64 Packages                                   
  404  Not Found [IP: 202.38.95.110 443]
Ign:88 https://mirrors.ustc.edu.cn/ubuntu bionic-backports/universe arm64 Packages                               
Fetched 62.4 MB in 8s (7,364 kB/s)                                                                               
Reading package lists... Done
E: Failed to fetch https://mirrors.ustc.edu.cn/ubuntu/dists/bionic/main/binary-arm64/Packages  404  Not Found [IP: 202.38.95.110 443]
E: Failed to fetch https://mirrors.ustc.edu.cn/ubuntu/dists/bionic-security/main/binary-arm64/Packages  404  Not Found [IP: 202.38.95.110 443]
E: Failed to fetch https://mirrors.ustc.edu.cn/ubuntu/dists/bionic-updates/main/binary-arm64/Packages  404  Not Found [IP: 202.38.95.110 443]
E: Failed to fetch https://mirrors.ustc.edu.cn/ubuntu/dists/bionic-backports/main/binary-arm64/Packages  404  Not Found [IP: 202.38.95.110 443]
E: Some index files failed to download. They have been ignored, or old ones used instead.

sudo dpkg --remove-architecture arm64