I have a fixture that uses one 36" T12 lamp. The recommended replacement ballast is a GE140RS120DIY. I am unable to find that ballast. I purchased a GE240RS120DIY ballast and wired it in. I wire nutted the two yellow wires with one nut to a wire. The new ballast is for two 48" lamps vs. one 36" lamp. It won't light and I'm wondering the reason why. Is there a way to wire it so it will work? Right now I'm two red to one end, and two blue to the other end of the 30 watt lamp.
Many ballast can be connected to 1 or 2 lamps some different lengths. The wiring may be different than your original (colors) look at the picture on the front of the ballast if it only shows connections for 2 lamps it probably will not work with just 1 lamp. This ballast should work found on line 13$. Fulham Fluorescent Electronic Ballast 120V for for (1-2) F32T8 (2) F25T8, F32T8, F25T12, F30T12, F3... by Fulham
You're wiring it contrary to its wiring diagram (on the label and on the spec sheet, found by googling GE240RS120DIY (you misspelled the part number). A single F30T12 is also not supported by that ballast.
Look for other ballasts which will support that bulb such as the GE240RS-MV-N.
You can also look for ballasts which take T8 bulbs. T8 bulbs will physically fit in T12 fixtures, however they do need a matching T8 ballast.
Edit: BETTER YET, get an LED "tube" replacement. It looks like a fluorescent tube, but it's made of LEDs. Here's the gotcha. Some of them, you rewire the fixture to put 120v directly on the pins of the ballast (called direct-wire). Others are "plug and play", meaning they cope with the weird voltages that come out of a ballast, and expect you to leave the old ballast in place. This is marketed as a time-saver, but is worse than useless if your goal is to get rid of the ballast entirely. You want 'direct-wire' if you go that way.
Your fluorescent tube has 2 pins on each end. Look closely at the sockets aka lampholders aka "tombstones". There are non-shunting and shunting types. A non-shunting lampholder has 2 pairs of stab connections, each pair goes to one pin on the bulb. A shunting lampholder allows only 1 pair, and shunts the two pins together internally. Rapid and programmed start ballasts use non-shunting, that is, 2 wires to each lampholder. Instant start ballasts use shunting lampholders (or non-shunting that you have jumpered) and have 1 wire per lampholder. If your lampholders are shunting, and your ballast supplies 2 wires to each tube end, you need to replace them with non-shunting. This is no big deal, they are pretty standard and cost between 60 cents and $3. A direct-wire LED conversion may require a non-shunting lampholder on one end or both.