According to your earlier statement, you have a KSU & 2 station modules & a trunk module. If you have 68 extensions including voicemail, you only have 64 station ports. Your 12x0 module supports trunks, not stations.
Go to Maintenance and check the ports one at a time starting at 101. Determine where your mods are plugged in to the 6-port expansion card, from 3 to 8 from bottom to top. The station modules should be in 3 & 4 (typically) and the trunk mod in 8, at the top.
Not knowing what software version you run, when in Maintenance, each port will show the DN, port and a lit LCD triangle after the DN if the port is 'equipped'. When you see the triangle, press the 'Device' softkey to show what kind of set is on the port.
Starting at port 101, press next to show each port through 132, 301 to 316, 401 to 416. When you show the trunk module it will show something like 801 L049, indicating Line 049, or the first trunk in the trunk module for port 8 (again depending on the software version).
I apologize for being so verbose, but it sounds to me like you have no ports left. If you do, you will find them in Maimtenance.