is that normal to have 1.5 as clear airway index?

Does it mean that I have central apnea that cannot be treated by pressure setting?

No, not at all.
Having a few centrals/clear airway apnea events is actually normal and doesn't mean that there is any sort of problem.
First the machine can and will flag some awake/semi awake breathing irregularities as some sort of apnea event and often it flags them as centrals. The machine doesn't know if you are awake or not and it just measures air flow. When we are awake or semi awake our breathing is much more irregular and we just don't notice it but the machine does
Second...it's normal to have what we call sleep onset or sleep stage transition centrals and those aren't a problem unless present in high numbers or they keep bouncing you out of sleep and preventing the normal progression into each sleep stage.

Even if all that 1.5 index was for sure asleep centrals that still not nearly enough of them to warrant a central sleep apnea diagnosis.

It seems that often people who are new to cpap therapy seem to spend an extra long bit of time awake/semi awake on the mask/machine and it's fairly common to see more centrals simply due to the awake/semi awake breathing irregularities getting flagged. Usually with time and as you get used to this therapy and fall asleep quicker and have less time awake on the machine these numbers will reduce but they may never totally go away.

You can use the software and check to see just exactly when those CAs are getting flagged and if they are real close to a known awake time then there's a good chance that they are awake/semi awake breathing getting flagged by mistake and can thus be totally ignored.

Sometimes when the OA count is high the CA count can be high IF the OAs are causing a person to wake up often and thus increase the chance of having sleep onset centrals when a person goes back to sleep.
So in those situations we work on fixing the OAs/hyponeas and see if the CAs happen to reduce.

Your 1.5 CA index..is nothing to worry about even if every single one of them was the real deal and thus nothing to worry about.
For therapy evaluation purposes you have to mentally remove them from the pressure needs evaluation process since we can't fix CAs with pressure unless using one of those high end specialty ASV machines.

+1 for Pugsy

With a caveat. If your CAs last a long time (30 seconds or longer) then it would be useful to discuss them with your doctor.
Longer centrals apneas will generally wake you up eventually.

Before I switched to an ASV machine, mine were in the 45 second range with the longest at 150 seconds (2.5 minutes!!). BTW the long one didn't wake me up and the SpO2 didn't drop significantly. Looking at the breathing graphs there was just the really long flat line with no breathing going on at all