The regular production ARC version only works with IPv4 addressing, which requires port forwarding. One way to configure port forwarding is automatically by UPnP (or else by setting up port forwarding manually)
However, Starlink uses IPv6 at its core and relies on CG-NAT (Carrier-grade Network Address Translation) to provide an additional limited IPv4 address for your internet access. These are sufficient for web browsing and the like when connecting to servers that use IPv4 and have not implemented IPv6 yet. However, these CG-NAT limited IPv4 addresses do not support port forwarding, so the regular ARC cannot work with CG-NAT.
The new test version in earlyaccess is able to use IPv6 addressing and does not require IPv4 addresses and hence port forwarding, and therefore is not affected by the CG-NAT limitation.
However, IPv6 does not use port forwarding and hence UPnP at all, so it is weird if you still get an UPnP error message with the earlyaccess ARC. This might be just a configuration issue on the router. But also see my next post below.
There is some more info about what the earlyrelease version provides in this regard here: