I am trying to reinstall OSX Mountain Lion but it says select the disk where you want to install OSX and nothing shows up to select

The reason nothing appears on which to install Mountain Lion is later macOS versions reformatted the startup disk using a disk format Mountain Lion does not recognize.

You need to follow these instructions: Use Disk Utility to erase an Intel-based Mac - Apple Support

Just prior to Step 4, you need to select the View menu > Show All Devices. Then, you need to select the internal storage device. "Device" in this context means a hardware component with an immutable name such as "APPLE SSD... MEDIA".

That's what you need to select, and erase. After that, you can quit Disk Utility and return to the Install OX X Mountain Lion screen. The disk should appear.

Try booting into recovery mode using Command + Option + R instead. This will access a newer version of an online macOS installer which may work better since Apple has been having lots of issues with the really old online installers recently.

Unfortunately older versions of macOS from 10.6 to 10.10, you must actually partition & format the drive first since these older versions of macOS do not understand the newer drive layouts & file systems utilized by later versions of macOS. Here is an article with instructions for partitioning & formatting the whole physical internal drive:

https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10_R3.pdf

If you are able to boot to a later version of the macOS online installer (macOS 10.13+) using Command + Option + R, then you can use @John Galt's instructions for erasing the drive with Disk Utility. The "View" option in Disk Utility was not implemented until about macOS 10.13+. Before that the physical drives were always visible on the left pane of Disk Utility.