I am using this guideline to install solartherm: https://github.com/SolarTherm/SolarTherm/wiki/Running-SolarTherm-on-Windows-%28MSYS2%29 I reach the point where I need to install Dakota and I get this error:
-bash: unzip: command not found
cp: cannot stat 'dakota-6.14.0.Windows.x64/*': No such file or director
The first error message there is telling you that unzip does not exist. That command comes from a file (/usr/bin/unzip.exe) that is provided by an MSYS2 package named unzip. So you need to install that package by running pacman -S unzip and then try again.
Note that the wiki page you linked to already told you to install the unzip package so I recommend that you go back to the beginning of the instructions and double check all of them to make sure you are on the right track. Pay attention to any error messages you receive along the way.