Update requested... taking too long. How to fix it?

First, ensure that your iPad is either connected to its Power Adapter (recommended) and/or the iPad battery is charged to 50% or greater - as this will be required for successful installation of the system update.

Next, force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

These support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

You need to be connected to WiFi which, according to that screenshot, you’re not. This is in addition to needing to be connected to a charger if you have less than 50% battery, as pointed out by LotusPilot.