Lack of memory in the iPhone storage

Find the Health App on your iPhone, Maks2609.

Long-touch the Health App: a contextual menu should pop-up.

There should be an option to Delete App, probably at the top, in Red.

That should “do the trick”.

You can also go to Settings -> General -> iPhone Storage, as you have done. Click on the Health App, and you should be presented with options to Offload App, Delete App, or to delete various files belonging to the App.

Welcome, Maks2609, to Apple Support Communities!

«it says that it takes 21 PB. What does this mean and how to deal with it?»

What it means is that the horribly incorrect—actually, impossible—storage space “report” by the Health App is causing significant trouble for your iPhone!

I expect that you cannot backup your iPhone: not to iCloud nor a computer.

Can you delete the Health App, whether along with it’s data or not?

I was afraid that this might be the case, Maks2609: the Health App is considered to be a part of the iOS, and, hence, cannot be removed. (That doesn’t explain why you are not permitted to delete its data, or even a portion thereof!)

This means that resetting to factory conditions, and restoring from Backup is the only option. (Especially if you cannot back-up, since the iPhone is “convinced” it has this impossibly huge amount of data.)

However, before doing that, I highly recommend you contact Apple Support directly (such as via the Contact Support link at the upper-righthand corner of this window)!