EF Core add-migration error

I am doing code-first migration. My command is

Add-migration -Name AddMyVariables

There are some error messages displayed as follows.

To undo this action, use Remove-Migration.

Add-migration : Exception calling "AddFromFile" with "1" argument(s): "User canceled out of save dialog (Exception from HRESULT: 0x8004000C (OLE_E_PROMPTSAVECANCELLED))"

At line:1 char:1 + Add-migration -Name AddMyVariables

• ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  • CategoryInfo : NotSpecified: (:) [Add-Migration], MethodInvocationException
  • FullyQualifiedErrorId : COMException,Add-Migration

    ---

What does the message mean?

The problem was found. The project file, *.csproj, was not checked out from code repository and non-writable. Once the file was checked out, the error message was gone.