Treadmill recalibration is not possible

You can delete the calibration data from the sensor menu.

Let us know whether the HRM-Pro pace is accurate after you have manually calibrated it. It didn't work for me.

Thanks for advice. Unfortunately I do not find such as Calibration in the sensor menu. BTW I am looking forward the recalibration option as it is written in the manual. 

Sorry, I jumped to the conclusion you were using the HRM Pro because I am dealing with HRM Pro calibration issues...

Yes, this is weird. The manual says the treadmill calibration should should show up.

Did you check out this article?

https://support.garmin.com/en-US/?faq=lmBMsI2lTR6JjUHbX91fLA

According to the manual, you should be able to calibrate after each run, but the distance on the watch (not on the treadmill) needs to be at least 1mil or 1.5km.

When you say you stopped after 2km, was it the distance of the treadmill, or on the watch?

More than 1.8km. I've just tried today recalibrating my HRM-Pro. Just to be sure 2km. 

I've noticed it works fine for me on the treadmill activity(after 2 runs it's calibrated although it struggles when changing the pace(l..it corrects itself but the instant pace is not as fast as the treadmill one) but on the virtual run activity it's impossible to correctly calibrate it. It shows me always the same pace after 3 or 4 runs. It doesn't change it.

As I initially described I could claibrate at first time, as Epix showed it up. But it was bad, now I cannot recalibrate. What you cited I knew and followed but inspite of it the calibrate+save option is missing I can only save, dicard etc. I think it is a bug, but I am newbie and wanted to ask whether it is so.
BTW the problem is that distance differs a lot. And during the run I checked also the pace on Epix which was much better than on treadmill. By my feeling and experience the correct value is treadmill.  

You should have lower in menu calibration option, at least on Fenix 7 it is always there.
You should stop activity, scroll down to see calibration, do this and then save workout.

Just before starting the run check if in the HRM-Pro settings the pace&distance menu is active and it shows indoor. I've noticed that sometimes the menu is not activated when the hrm-pro connects(this bug doesn't happen with the hrm-plus)

Yes I scrolled down, but the problem is that there is no calibration...This is what I claimed/asked for.

I first calibrated as you on My Epix. Then run a second time using Treadmill exercise and after that calibrated by doing the following. Stopped the recording (but did not saved), then scrolled down in menu and entered "Calibrate" selected a new distance, then return to previous screen and saved exercise 

I tried the same, but when scrolling donw there is no "Calibrate". This is the problem I try to explain.