The Campaign Creation screen won't let me list Battletech A Time of War as our system

I can enter Battltech and Battletech RPG as keywords but in the "playing" dropbox I can not enter Battletech

There is a limited selection of some of the most-played games in the "Playing" dropdown. Roll20 adds to the list from time to time, when they find another game rising in popularity with frequent listings on LFG. For an unlisted game, use "Other Games" in the dropdown, and then use your game title in the Keywords and use the title in the Description text (preferably at the start, within the first few words of your description, so it shows up in the excerpt). A couple other generic/encompassing categories you can use in the dropdown, if they apply to your game: "Story games", "Old School Revival", but "Other Games" is the most common general category in the dropdown.

You would need to choose Other Games for Playing, if you want to set something. Using Battletech for a keyword and in the description of your game will allow individuals to find you through the looking for group system. This is not a Bug or Technical issue. The list of games is mostly based on the volume of games played on Roll20. There are simply too many game systems out there to list them all. Game systems which have a steady stream of games being played on Roll20 are often added to the list of Games.