How do I debug my PIC16F15323 code?
Maybe your MPLAB-X installation is corrupt or very old. You should be able to select the assembler where it asks you to select a compiler.

As you can see, versions at least 4.01 and newer support the chip you have in mind, and 3.4 does not.
You may have to install the free version of XC8 to get the assembler. I think at some point in the past they combined the two.