Hi,
You can refer to the following troubleshooting steps to solve your issue: https://mathworks.com/matlabcentral/answers/112533-why-am-i-still-being-prompted-to-close-all-matlab-sessions-during-an-installation-process-after-i-ha.
Since you have already checked task manager to verify if MATLAB is running or not, I would recommend restarting the system would hopefully solve your query.
