$Tesla, Inc.(TSLA.US)$ CEO Elon Musk tweeted out Saturday the latest version of the company's autonomous driving software isn't quite ready for prime time yet. It's taking longer to deliver the higher functioning version of Tesla's self-driving system than Musk expected. More delays risk becoming an overhang for the stock.

Tesla ships its vehicles today with a system called Autopilot, which includes its basic driver assistance features. For an additional charge -- about $10,000 -- Tesla drivers can upgrade to what the company calls Full Self Driving, or FSD, which has more driver assistance capabilities.

Tesla investors have been waiting for the company's version 9 of its FSD software. Musk tweeted in late April, the software would be ready in about two weeks. The tweet Saturday, indicating things were taking longer than expected, didn't include an updated timeline.

Delays are a problem, but at least investors can take solace in "shipping soon." Autonomous driving, after all, isn't easy, and autonomous driving systems are getting more advanced. Drivers can purchase assistance systems on many new cars, including Tesla vehicles, which have features such as automatic emergency braking, lane keeping assist, parking assistance and adaptive cruise control, among others.

Software is at the heart of all of these new features -- software and onboard computers with the power to integrate camera and radar sensor data and make decisions for the car. Today's systems don't create fully self driving cars yet. Drivers still need to watch the roads. But today's features are part of the advances leading to self driving passenger cars.

Leading the way in self driving is important for any auto maker, including Tesla. Autonomous driving features essentially lead to more money when customers purchase them. Autonomous driving features can also be bought via subscription, giving any car maker high margin, recurring revenue. That type of sale isn't typical for car companies.

Tesla investors have also been waiting for a subscription option for FSD to roll out as well. That also is taking a little longer than expected.

Tesla could be planning to rollout its subscription payment plan when version 9 of the company's self driving software is ready. Tesla didn't return a request for comment on the holiday Monday, July 5.

Delivering the software could be a catalyst for Tesla stock. After version 9 ships, investors will pay close attention to the number of subscriptions bought in coming quarters. More delays, however, could create a near-term headwind for Tesla shares.

Tesla stock has paused after an epic 2020. Shares are down about 4% year to date, trailing behind comparable gains of the S&P 500 and Dow Jones Industrial Average. Shares, however, rose 743% in 2020, making Tesla the world's most valuable car maker.

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