SQL AlwaysOn asynchronous node backup

Hi,

I have three (A, B and C) servers with AlwaysOn.

Two servers are synchronized and I have another one in azure that is asynchronous.

I have AG1, AG2, AG3 and AG4.

AG1 is the backup preference is the secondary the primary server is server A

AG2 is the backup preference is the secondary the primary server is server A

AG3 is the backup preference is the primary the primary server is server B

AG4 is the backup preference is the primary the primary server is server B

PS: In AG4 there are bases that only have it and server B e C.

Server C receives all AGs, but it has the option (Exclude Replica) selected.

When I try to make a backup of the databases on server C it doesn't work.

What is wrong?

Thanks.