I've downloaded the main .jar file but it seems that the class is missing or not connecting. what do I do?

PS C:\WINDOWS\system32> java -jar "C:\Users\Eagle eye\Downloads\BBOutlineReloaded-2.2-1.16.1-fabric.jar"
Error: Could not find or load main class com.irtimaled.bbor.Main

Not sure why you are trying to open the fabric version by double clicking on it. Just drop it in the mods folder and open fabric 1.16.1