failed to fetch ...

1. Support for Ubuntu 12.04 (precise) has ended a month ago (see https://wiki.ubuntu.com/Releases). You should consider a release upgrade in due course, because all repositories for precise will be removed soon.

2. If you repeat that command, do you consistently get the same error? Maybe it was just a temporary problem.

What you are doing isn't dissimilar to searching for Windows updates and you are running Windows 2000.

They don't exist. Ubuntu releases don't last forever.

Hi, I have changed my computer system from Ubuntu 12.04 to Ubuntu 16.04.

And when I run these commands in the terminal:
sudo bash -c 'echo "deb http://www.yade-dem.org/packages/ xenial/" >> /etc/apt/sources.list'
wget -O - http://www.yade-dem.org/packages/yadedev_pub.gpg | sudo apt-key add -
sudo apt-get update
All of them is ok.

But when I input "sudo apt-get install yadedaily" in the terminal, it emerged some problems:
raymong@raymong-Lenovo:~$ sudo apt-get install yadedaily
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package yadedaily is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'yadedaily' has no installation candidate

Now what can I do for it? Thank you.

For diagnostic purposes please provide the full output of the commands

tail /etc/apt/sources.list
sudo apt-get update
apt-cache policy yadedaily

Hi, Manfred
Thank you so much for the answering.

raymong@raymong-Lenovo:~$ tail /etc/apt/sources.list
# deb http://archive.canonical.com/ubuntu xenial partner
# deb-src http://archive.canonical.com/ubuntu xenial partner

deb http://security.ubuntu.com/ubuntu xenial-security main restricted
# deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://security.ubuntu.com/ubuntu xenial-security universe
# deb-src http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse
# deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse
deb http://www.yade-dem.org/packages/ xenial/

raymong@raymong-Lenovo:~$ sudo apt-get update
[sudo] password for raymong:
Hit:1 http://cn.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://cn.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:3 http://cn.archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:4 http://www.yade-dem.org/packages xenial/ InRelease
Get:5 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Fetched 102 kB in 8s (12.6 kB/s)
Reading package lists... Done

raymong@raymong-Lenovo:~$ apt-cache policy yadedaily
yadedaily:
  Installed: (none)
  Candidate: (none)
  Version table:

Try

sudo rm -v /var/lib/apt/lists/*yade*
sudo apt-get update
apt-cache policy yadedaily

and report the results.

And please provide also the output of

uname -a
dpkg --print-architecture
dpkg --print-foreign-architectures

raymong@raymong-Lenovo:~$ sudo rm -v /var/lib/apt/lists/*yade*
[sudo] password for raymong:
removed '/var/lib/apt/lists/www.yade-dem.org_packages_xenial_InRelease'
removed '/var/lib/apt/lists/www.yade-dem.org_packages_xenial_Packages'

raymong@raymong-Lenovo:~$ sudo apt-get update
Hit:1 http://cn.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://cn.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:3 http://cn.archive.ubuntu.com/ubuntu xenial-backports InRelease
Get:4 http://www.yade-dem.org/packages xenial/ InRelease [2,460 B]
Get:5 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Get:6 http://www.yade-dem.org/packages xenial/ Packages [2,192 B]
Get:7 http://security.ubuntu.com/ubuntu xenial-security/main i386 DEP-11 Metadata [54.6 kB]
Get:8 http://security.ubuntu.com/ubuntu xenial-security/main DEP-11 64x64 Icons [41.9 kB]
Get:9 http://security.ubuntu.com/ubuntu xenial-security/universe i386 DEP-11 Metadata [35.7 kB]
Get:10 http://security.ubuntu.com/ubuntu xenial-security/universe DEP-11 64x64 Icons [52.2 kB]
Fetched 291 kB in 17s (16.4 kB/s)
Reading package lists... Done

raymong@raymong-Lenovo:~$ apt-cache policy yadedaily
yadedaily:
  Installed: (none)
  Candidate: (none)
  Version table:

raymong@raymong-Lenovo:~$ uname -a
Linux raymong-Lenovo 4.8.0-36-generic #36~16.04.1-Ubuntu SMP Sun Feb 5 09:39:41 UTC 2017 i686 i686 i686 GNU/Linux

raymong@raymong-Lenovo:~$ dpkg --print-architecture
i386
raymong@raymong-Lenovo:~$ dpkg --print-foreign-architectures
raymong@raymong-Lenovo:~$

oh, my god. ok, i will install new version. thank you for your help.

Why did you not install the 64bit version anyway, or is your system ancient?

Thank Manfred, that solved my problem.

actionparsnip, when I used windows, I used 32-bit processing, so I installed the 32-bit version ubuntu directly when I chose ubuntu as my new operating system.

Thanks Manfred Hampl, that solved my question.