Apple Mail App :: "Erase Junk Mail" is greyed out
Go to mail>preferences>junk mail>under when junk mail arrives make sure options #2 move it to the Junk mailbox is selected and this should fix your problem immediately. 

Let me know if you need any help, this is what worked for me after I let my junk mail build up to over 600, which was killing me, I can understand you want it gone! Should look like this: