"Could not be read" means that you do not have WRITE permission to the destination directory, so change your permissions.
"Another application already has it opened" can be fixed by deleting the .lock file in the same folder as your catalog file.
For the second problem being that the catalog is open in another application, go to the folder containing your catalog. You will find a file with the .lock extension. This is a file created by the catalog, and is automatically deleted when the catalog closes normally. Sometimes it can get left behind. You need to delete that file.
When Lightroom says that the file cannot be read, it usually means that it cannot be written to the chosen destination. This tends to be a permissions problem in your operating system. Check to make sure you have write permission for the folder you are trying to import your images to.
Thank you. I used a solid state drive on a main computer and inadvertently placed a large file on there so there was not space for LR to import. When I changed drives, the import worked again.
Thank you. I had over loaded the drive to which I was importing and there was insufficient space for the import. When I changed drives, it worked fine.