According to new legal docs, obtained by TMZ, the judge says the court lacks the legal authority to grant the request ... so his hands are tied. But, get this ... even though the judge's hands are tied, he implies Tekashi might want to take his chances and file his request instead with the Bureau of Prisons.

Tekashi's attorney, Lance Lazzaro, may do just that cause he tells TMZ he believes the judge's decision "is a strong recommendation to the BOP to release him immediately."

The judge also explained had he known at the time of sentencing there would be a global pandemic and Tekashi's health history made him vulnerable ... the court would have directed the final four months to be served in home confinement.

TMZ broke the story ... Lazzaro wrote a letter to the judge saying there was a serious concern for Tekashi remaining behind bars because he suffers from asthma, which would make catching the coronavirus that much more serious.