Why i cant open my online banking

First, try closing the app completely by swiping up to the center of the screen and swiping the app up and away.

If you still experience problems opening your online banking app, start by checking whether it is up to date. Find the App Store icon, long press and tap "Updates" in the pop-up menu.

It says in your description that you are on iOS 15. This could also be the reason why your online bank is not working. Many banks and high security apps often require users to update to the latest available software. In your case, you can update to iOS 16.3.1.

Finally, it could also be an error with your Online Bank. There you must contact them to get this clarified.

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

You can find more useful information here:

Update your iPhone or iPad - Apple Support