What can I do when I get this message: "This project uses legacy Windows support which is being phased out and will no longer be available for new projects in the next Enterprise version"

[Htet_Aung_Lwin]

Hello Marian,

After I migrated, I got many kinds of errors. Like these two.
There were no problems at my window legacy codes.

I cannot run any programs after migration. Nothing changed after I clicked yes or installed latest version of .Net from Microsoft.

I wrote with C# . I got so many errors like that in all activities.