cannot connect iphone 7 by bluetooth with ipad series 8

What, specifically, are you attempting to achieve by pairing an iPhone and iPad over a Bluetooth connection?

Whilst it is technically possible to do so, for the majority of applications, Bluetooth pairing of an iPad and iPhone is entirely pointless. If your intention is to use Cellular Hotspot features of your iPhone, this link is optimally achieved using the native WiFi hotspot connection - this being a largely automated process. Hotspot connections over WiFi offer substantially greater throughput, range and reliability.

If you are exploring one of the very few “edge-cases” where it is necessary to pair iPad and iPhone using Bluetooth, both devices must be simultaneously placed in Bluetooth Discovery Mode.

My pleasure to assist.

Missing from my earlier directions is the need to ensure that your iPhone Personal Hotspot is manually enabled prior to attempting the mutual pairing request (Allow Others to Join is set to ON).

Bluetooth pairing between iPad and iPhone is never straightforward - and is rarely necessary. Sometimes it takes many attempts.

Correct, as there is no supported Bluetooth protocol to do so. What are you hoping to accomplish? There is usually an easier method.

You’re welcome. I delighted to learn that your problem has been fully resolved.