Hi berket,
Thank you for your question.
We could refer to the following steps to check if the issue persist:
1) We went ahead and removed all the versions of .net using dotnetfx_Cleanup_tool by the following link:
http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
2) Then we rebooted the Exchange Server and then installed .Net 2.0 SP2 x64
https://www.microsoft.com/en-us/download/details.aspx?id=1639
3) Then we again rebooted the Exchange Server and tried installing the CAS role to check if the issue persist.
If there are any questions regarding this issue, please be free to let me know.
Best Regard,
Jim
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Jim Xu
TechNet Community Support
