I cant install ios 14.3 on my ipad

The tags appended to your post suggest that you have an iPad Air2 with iOS12 installed. If this is the case, then your iPad is capable of update to iPadOS 14.3. If instead you have an iPad Air1, your iPad can only be updated to iOS 12.5.1; in this instance, update to a later version of iPadOS is not possible.

Misidentification of iPad Air and iPad Air2 is not uncommon. You can check your model number and currently installed iOS version here:

Settings > General > About

Not sure which iPad you have? Identify your iPad model.

If you do have an iPad Air2 or other model capable of update, available methods by which you can update your iPad are described here:

Update your iPhone, iPad or iPod touch – Apple Support

If your iPad is capable of update, but you are experiencing difficulties, these support pages may be helpful with troubleshooting the issue:

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

If you need more space for an update - Apple Support