The clue is in the DTC translation, quite simply the TCM "knows" the ratio of every gear and can monitor them by a simple input vs output speed signal comparison, so a failure by the gearbox to achieve and then maintain an expected ratio will set this code. The only time this code can be caused by a control system fault is if the final drive ratio's been altered, ie; a diff swap or conceivably a mismatch in the TCM/ ESP/ABS module coding ie; wrong module(s) installed.

The only realistic external check available is the fluid level, low fluid = low hydraulic pressure = clutch and brake slip, note that the fluid level on these is checked with the engine running! If that's ok then it's fair to assume that the box has an internal component failure and needs a stripdown to to assess it further. HTH.