A kid I once knew got a job and was all jazzed up about saving his money to buy some ridiculous thing that only a kid could want. He asked me "can you take me to a store where I can buy a piggy bank?" I asked him why he wanted to spend the money he had earned on a piggy bank, rather than saving it for his dream toy. He replied "I have to have a piggy bank so I can save my money!"
I have avoided writing because I had to do research first, or I didn't have access to a computer, or the computer I had access to only had Lotus and have to have Word!
Here's where I would typically trot out a long anecdote about how J.K. Rowling writes long hand, or how difficult it was for Shakespeare to pen his five-act epics on crappy parchment with crappy quill feathers by the crappy light of a crappy candle.
Instead, let me offer a practical piece of advice for turning the "have to have" rut around, and getting something done: Assess what you do have. See what can be done with that.
The thing that always draws me back to the theatre is the "make-do" aspect of the art. I like solving problems and getting all MacGyver. It's all about getting by on what you got. In this world of plenty that we live in now, it's easy to get accustumed to always getting exactly what you want when you want it. That's cool ... but not getting what you want should never prevent an artist from creating.
Take what ever it is you're lacking and use that to your advantage. Don't have access to a computer? Pull your play in a pulp fiction/noir-ish direction, and hammer the thing out on that manual typewriter your parents keep in the garage. Maybe seeing your words spill out across a slice of onion paper coiled around the platen of a monstrous old Royal will inspire you to melodramatic heights not reached since the days of Sam Spade.
Why not write the thing in crayon on butcher paper? Who cares? Just write. If someone says "hey, why is this play in crayon on butcher paper?" Tell them "you'll have to read it to find out."
Need to do research first? Go to the library. Or here's a thought: write something that doesn't require research. Or make up the research. I'm picking away at "The Worst Play Ever Written [working title]" that makes a big point of made up research.
Don't get discouraged, just put one word after the next. That's more than most folks ever manage to do!