Hello Mkamaraj and thx a lot!
yes it helped, it confirms what I though since I tried different things and couldn't make it working.
I'll do the update to the version that supports validate, that's a good tip!
Regardaing the link to the upgrade paths, I read all of that first of all but eventually it was still unclear if i could go on my (production
SRX550M clusters directly from 15.1X49-D30.3 to 17.3. Now the question is from 15.1X49-D40 to 17.3