At this time, the QRadar Threat Intelligence app only supports STIX/TAXII 1.2. I know this is a change that is on the roadmap and dev is reviewing these changes now. You will likely need to use something to downgrade STIX to v1.2 for now.
You might be able to use a slider to convert the feed or see if the feed has a 1.2 format: https://pypi.org/project/stix2-slider/
I've asked development if there is a work item that you could be added to if you wanted to open a case, but you are going to need to use a 1.2 feed as STIX2 is not available in the Threat Intel app yet.
#QRadar#Support#SupportMigration