The small black window in the center of your screenshot indicates that you must have manually added at least one non-system repository, which doesn't have proper release file, suitable for your current system (apparently, Ubuntu 18.04 if you're on elementary OS 5).

To solve this issue, you should remove that particular PPA from your system:

1. Right click on Files and select New Window as Administrator.
2. Go to /etc/apt/sources.list.d folder.
3. Inside that folder, delete all files that contain sarimkhan in their names.
4. Then try running an update via AppCenter.

In case of more errors (the messages that begin with E:) post them here.

The small black window in the center of your screenshot indicates that you must have manually added at least one non-system repository, which doesn't have proper release file, suitable for your current system (apparently, Ubuntu 18.04 if you're on elementary OS 5).

To solve this issue, you should remove that particular PPA from your system:

1. Right click on Files and select New Window as Administrator.
2. Go to /etc/apt/sources.list.d folder.
3. Inside that folder, delete all files that contain sarimkhan in their names.
4. Then try running an update via AppCenter.

In case of more errors (the messages that begin with E:) post them here.

# Or please execute this command bellow in terminal to remove that repository,

sudo add-apt-repository -r "deb http://download.opensuse.org/repositories/home:/sarimkhan/xUbuntu_14.04/ ./"