ipad Air 2 , can it be updated from 11.4 to newest 15..?

Your iPad Air2 supports iPadOS15.x - including the most recent update to iPadOS 15.3.1.

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

Be aware that your iPad may be too far behind the current version to be updated directly from iPad settings. As such, you will likely need to use a computer to perform the update. You will need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

iPadair2user1 wrote:

https://discussions.apple.com/content/attachment/b0d42f03-5912-4895-a626-a7191da3d379

Older ipad air 2 can it be updated from an older ios ?

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

An FYI, if you were not able to install the iOS on your device you would get a message stating that your software is up to date. The fact that you were able to download the software, means it is compatible with your device.

Try deleting the software and download it again. That is the very last suggestion in the support article that razmee209 posted for you.

Your screenshot would suggest that you’re not on WiFi, which might be what the problem is.

Glad to hear that you got it resolved.