It is not able to add HA secondary without High Availability Appliance type. Your secondary should installed as High Availability Appliance then able to add your host as secondary.
Installing a QRadar appliance
https://www.ibm.com/support/knowledgecenter/SS42VS_7.4/com.ibm.qradar.doc/t_siem_inst_con_man_host.html
For your secondary host installation, you need to select High Availability Appliance in step 3.
Step3. Select the appliance type:
High Availability Appliance
#QRadar#Support#SupportMigration