A web browser process is still active in the background.
Please make sure that all web browsers and web browser background processes are closed before you continue with the installation.
Use CTRL+ALT+DEL to open the Task Manager.
There, click on "More details" at the bottom left.
You will now find all processes in the "Name" column. It is possible that a process of your web browser is still running there, even if it appears to have been closed.
Click on the process and then click on "End Task" at the bottom right.