On Debian 9 I want to install the package libicu57 (repository), but for some reason I can't:

$ sudo apt install libicu57

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Package libicu57 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 'libicu57' has no installation candidate

In my /etc/apt/sources.list is this (beside others):

deb http://ftp.cvut.cz/debian/ testing main non-free contrib

What is the problem and how to solve it?

Issue at Hand

You wish to install libicu57 from the testing repo. However, if you are using x86 or amd64 you cannot install this package as it is only available in testing for armel mips and mipsel.

You have in your sources.list an entry for Debian Testing (Buster or 10). You report that you are using Debian 9 (Stretch or Stable). You have created FrankenDebian. You need to configure your sources.list properly remove this entry and rollback or remove any packages you have installed from the testing repo to correct this.

After you have done this, if it is the ARM variant of libicu57 that you require you can look into cross compiling solutions or better yet virtualization.

Fix sources.list

First step is to verify that your repositories to your /etc/apt/sources.list are good. I would recommend you use /etc/apt/sources.list.d/ where each repository is it's own file, but you can do this in just one file.

#stable.list

deb http://deb.debian.org stable main contrib non-free 

deb-src http://deb.debian.org stable main contrib non-free



#updates.list

deb http://deb.debian.org stable-updates main contrib non-free 

deb-src http://deb.debian.org stable-updates main contrib non-free 



#security.list 

deb http://deb.debian.org/debian-secruity stable/updates main contrib non-free

deb-src http://deb.debian.org/debian-secruity stable/updates main contrib non-free

Your sources.list should look something like this. You can change the exact http address to match your desired mirror. If you must absolutely have a testing repo, add the entry in a similar manner as above and use apt preferences to maintain some level of sanity. Apt-pinning should be used sparingly as it can lead to the creation of FrankenDebian.

Install software

Now all that is left is installing your package, which can be done with this command:

 apt-get install libicu57

or

apt-get install -t stable libicu57 **or** apt-get install libicu57/stable

You can also choose to install the package you desire from source or download it and its dependencies from the Debian Packages site and install them with dpkg -i [packageName].deb

This should install libicu57. Please comment if there is another error and we con look further into solutions. The next part is going to cover if you absolutely require the ARM or MIPS versions of software you can look into cross-compiling, or creating a virtual environment with the appropriate instruction set to safely use this version of the library.

Here is the Debian Wiki entry on Multiarch and here is the Wiki for cross-compiling. You can follow these steps to make the appropriate configuration changes to dpkg and what not to cross-compile.

However you cannot have multiple versions of the same software installed!

So if the ARM version requires dependencies and libraries that you already have a version of in x86 or amd64 then you cannot do this. The alternative is to create a virtual machine of Debian for ARM using the virtualization solution of your choice that uses the correct instruction set.

KVM is a good choice but you can use whatever solution best fits your use case.

Conclusion

Please read over this in its entirety. In fact, please make sure you read over all the links I have provided thoroughly before running any commands. If you have any questions or concerns about this post, do not hesitate to ask me. If there are any corrections or misconceptions in this answer please inform me. I can update the post as necessary.

Best of Luck!