Try to edit envrionment Variables.
Right click on This PC -> Properties -> Advanced System Settings -> Advanced -> Environment Variables -> System Variables
Add [install path] to the variable path.C:\Program Files\dotnet\
Restart visual studio.
My answer is from this case, you can refer to it https://stackoverflow.com/questions/65209536/microsoft-visual-studio-2019-the-project-file-cannot-be-opened-unable-to-locat
https://superuser.com/questions/1431833/visual-studio-2019-unable-to-locate-net-core-sdk

Try to edit envrionment Variables.
Right click on This PC -> Properties -> Advanced System Settings -> Advanced -> Environment Variables -> System Variables
Add [install path] `C:\Program Files\dotnet\` to the variable **path**.
Restart visual studio.
If it does not solve this problem, you can refer to this [answer][1].

[1]: https://superuser.com/questions/1431833/visual-studio-2019-unable-to-locate-net-core-sdk