A new tweet from Elon Musk suggests that the all-electric Tesla Semi may not launch in 2021 as originally planned.

The Semi was unveiled in late 2017 and while Tesla had intended on commencing deliveries in 2019, that date came and went without any units being shipped. As of recently, it was understood that some deliveries would be made before the end of 2020 but that no longer seems to be the case.

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While responding to a tweet about how the Tesla Semi is coming along and by what year the automaker will release its semi-autonomous, all-electric truck with an innovation platooning feature, Musk suggested it may not happen until next year.

“We are too cell-constrained right now, but probably okay next year,” he wrote.

A key reason why the Tesla Semi is taking longer to become a reality than originally planned is the fact that it is tipped to use the brand’s new 4680 battery cells. Tesla unveiled these cells last year and is currently only producing them at a pilot plant at its Fremont factory. While it will soon begin to ramp up production and create several gigawatt-hours of 4680 cells in the near future, it will have to allocate some of these cells to other models, including the Model Y set to be manufactured at its Gigafactory in Berlin, Germany.

Initially, the Tesla Semi was quoted as offering 500 miles (804 km) on a single charge, but last year Musk stated that its range would actually be 600 miles (965 km).