Did you check whether "FlexConnect" mode is available ? H-REAP was renamed as "FlexConnect" since 7.2.x onwards. Once you convert it as FlexConnect AP you should able to see a "FlexConnect" tab under AP configuration.
I hope your AP model is 3502 (may be 3521 is typo ?)
Yes FlexConnect Mode is their. If i am Convert AP to FlexConnect then can I need to Configure remote Site VLAN ID in Controller to allow or MAP the Specific SSID to particular Remote Site VLAN Range. Is it possible.
Yes, I will do this activity, but in my controller i am having 11 AP's register. From that 5 is remote site and 6 is local AP's. If i am convert 5 AP's to FlexConnect is their any impact is happening to rest 6 local AP's. And in my two sites the three VLAN ID is same but IP range is different, so when configuring the WLAN on Controller the VLAN ID will create problem or not.
Thanks Rasika your help is really appreciated for me.