Failing to change the target framework

Hi ,
I am trying upgrade the csproj file to VS2017 format. I am following the steps suggested in this blog
https://natemcmaster.com/blog/2017/03/09/vs2015-to-vs2017-upgrade/ . I am trying to manually convert it. but facing the error attached.

Steps to reproduce:
1) Unload the project whose csproj file you need to change
2) Follow the manual upgrade steps stated in the above blog
3) Save the changes and reload the project

You will the below error. Not sure if this is occurring only for me

I assume the error is popping when I am trying to change the target framework from v4.5.1 to net451.
Would be glad if someone could help me out here.