Star Trek Original Series Tricorder
I don't think it qualifies as a tablet. . .
There were some scenes in Star Trek where an ensign would hand Kirk what appears to be a tablet and a stylus which he'd look at and sign or check off, but no one has really looked clearly at one, as far as I can recall. That may have only been a note book.
Perhaps the first "invention" of a fictional tablet TV is seen in Stanley Kubrik's 2001, a Space Oddysey:
They spent several tens of thousands of dollars to display what appeared to be two fictional tablet TVs playing on the ship going to Jupiter. . . and they actually did indeed have a real video images on them, because they did not have the post production means in 1968 to do that in post-production, but the FX people said they WERE indeed part of the desk they were sitting on and took a huge amount of electronics underneath the desk to drive the CRT screens to make them look like they were portable tablets, just idly laid down on the desk. Samsung tried to claim this was "prior art" and invalidated Apple's patents on the iPad. The court disagreed, as it did not show a computer tablet, but it only a TV images.