The original claim is "the Department of Justice investigation bluntly stated that he is not mentally fit" as reported by the video you linked from the TikTok account dingnewscorp.
With both the original video and the claim presented here, this claim can be found to be false with looking at the original source of the mentioned DOJ investigation. First, I would correct that the Department of Justice did not make any sort of statement where we can attribute where this claim stems from. Instead, we see an original document from February 2024 from the DOJ Special Counsel Robert Hur who was reporting on the investigation involving President Biden. In that original document, there are mentions of poor memory that is related to the recall of information that is relevant to the investigation. Ultimately, reading through the entire document, there is no direct statement or conclusion from this investigation that produces the DOJ to this wide claim of the President being "mentally unfit." I'd also argue that the TikTok account dingnewscorp is not reputable and they seem to provide a lot more of their take than needed in reporting and sharing information.
The original source was used in this factchecking which can be found here: https://www.justice.gov/storage/report-from-special-counsel-robert-k-hur-february-2024.pdf
We also see a news article who has their own take on this report here: https://www.usatoday.com/story/news/factcheck/2024/02/21/false-claim-biden-declared-mentally-unfit-trial/72669285007/
