I have a KX-TA824 and TVA50 but I can't get the voicemail to pickup only the Auto Attendant. Can someone tell me what I am doing wrong.

Please explain what you want to do.
Call vm extension and it won't answer?
Call std extension and it does not go to VM?
Do you have the manuals? Comes with the equip.
Most likely just not programmed properly.

When someone calls the main number I just want the main voicemail box to answer and collect the messages.

Program the lines to DIL to the VM lead extension.
3.1 in maint console.
This would answer with Auto Attendant, caller would dial an extension or 1 digit from a menu, this would need further setup.

Or you could just DIL lines to main ext (101) and call fwd no answer to the VM ext. Phones would ring 4 times and then go to MB 101. A giant answering machine. Just record a greeting in box 101.