I've created a virtual environment named test_env I have been trying to add matplotlib using Anaconda Navigator this way

Then, it installs something. After installation finishes, it is like before installation again. I know, it is weird. But why it happens?

Also when I go to Anaconda Prompt and write these

activate test_env

conda install matplotlib

I see this error

ERROR conda.core.link:_execute(700): An error occurred while installing package
'defaults::qt-5.9.7-vc14h73c81de_0'.
Rolling back transaction: done

LinkError: post-link script failed for package defaults::qt-5.9.7-vc14h73c81de_0

location of failed script: C:\Users\mustafa\anaconda3\envs\test_env\Scripts\.qt-
post-link.bat
==> script messages <==
<None>
==> script output <==
stdout:         1 file(s) copied.

stderr: 'chcp' is not recognized as an internal or external command,
operable program or batch file.
'chcp' is not recognized as an internal or external command,
operable program or batch file.
'chcp' is not recognized as an internal or external command,
operable program or batch file.

return code: 1

()