A problem with the batteries is causing unwanted delays.
Tesla, or more specifically, Elon Musk, has been getting better about his habit of over-promising and underachieving. However, the Tesla Semi truck is still years overdue. It was first mentioned in 2016 as part of Tesla's "Master Plan." The company debuted the concepts in 2017, saying production would start in 2019. Then Tesla pushed it back to early 2020. We're now in 2021 with no production underway and with companies expecting their orders. When asked on Twitter last week, "How's Semi coming along, Elon? What year do you think we'll see follow-the-leader driverless platooning?" Musk replied, "We are too cell-constrained right now, but probably ok next year."
It appears that even the conservative timeline was too fast for Tesla as Musk has now clarified the issue, explaining, "Demand is no problem, but near-term cell supply makes it hard to scale Semi. This limitation will be less onerous next year."
Part of the reason for the conservative (for Tesla) timeline for the Semi was always the lack of battery production capability. Each Semi will need at least six times as many battery cells as the Model Y, which is understandable. That comment from Musk was regarding a report that Tesla had just received an order for ten trucks and two chargers from a California-based company.
However, orders go back four years now, and we know PepsiCo has 100 units on order and is expecting 15 by the end of this year. Musk's vagueness in his reply suggests that limited production may start, and PepsiCo might get its trucks. The truck will be using the Tesla 4680 battery cell technology unveiled on 'Battery Day' in September 2020. They are only being produced at its "pilot plant" in the Fremont factory at the moment, and it's unlikely there will be consistent production of the Semi until those batteries are produced in volume at the Gigafactory in Texas. The Model S Plaid+ is also set to use the 4,680 batteries and, predictably, has also been delayed.