yarn was previously provided by cmttest and these are very different programs to yarn the package manager.  If you are using software sources (e.g. FarmBot.io)  that are managed through yarn (you will find a yarn.lock file with the software source) but when you try and run yarn install and then you get e.g.

yarn install
ERROR: [Errno 2] No such file or directory: 'install'

then firstly check to make sure that cmdtest is not installed.

sudo apt remove cmdtest

You would install yarn with

sudo apt-get install yarn

See the yarn packager web site for more details on installation.