Why my ipad cannot install ios 5.4

You don't have enough memory available to do the update. You need to either delete a LOT off your iPad (photos, music, apps, email, messages, files, etc.) or buy an iPad with more memory as clearly the one you own doesn't meet your storage needs. At all times you should have at least 6GB of space available.

As to being plugged in, you need to be at least at 50% charge. It is recommended to be plugged in when you update your device.

I assume you mean iPadOS 15.4. Please answer the following question (all of them):

1. Going to Settings > Genreral > About - what is the model version listed?
2. Going to Settings > General > iPad Storage - how much free space do you have on your iPad?
3. Assuming you have an iPad eligible to update, what happens when you try and do you get any error message?
4. Are you plugged in to charger and connected to Wifi?