Value should be a JSON object about zabbix-template-juniper-mx-bgp4-ipv4-ipv6

[AmandaJangrossi]

Hey, how are you doing?
I knowit's a bit late, but I found the solution for this, if you are using Zabbix v5.XX or above, probably you gonna find two problems, in my case it gave me an error in the graphics prototype and the same as yours ("Not supported" and "Value should be a JSON object.") in the item key.
For the graph prototype the solution is just open the XML file in a notepad/code editor (I used VSCode) and remove everything is inside the <GraphPrototypes> tag

(On left it's the original template and on the right it's how it's supposed to be).

Next, to solve the issue that both of us had, you have to change some of the key sintaxe in your Zabbix Template. Access your JunOS BGP4 discovery rule, and on the key adapt like this:
LLD.py[-h, {HOST.CONN}, -v2c, -c, {$SNMP_COMMUNITY},"-mi", "{#PEERADDR}", "-m", "{#PREFXTBL}", "{#ADDRTYPE}", "{#ASNUM}"]
I assume that you already register you SNMP_COMMUNITY macro, but if not, just access Administration -> Geral -> Macros and set your community.
I highly suggest testing before accept, if everything it's fine it gonna come up like this:

I hope it helps!