Hi,
thanks for the error message! That is really strange. I don’t see how this could be happening.
I have now tested with PHP 7 as well and am not able to reproduce this. Also, this error is not really specific to PHP 7 and should have happened with other versions as well.
Is it maybe possible that your TablePress installation is corrupt? Can you please reinstall TablePress from a fresh download to make sure that all files exist?
Also, can you please try again after temporarily deactivating all other plugins and after switchting to a WordPress default theme like Twenty Sixteen? That way, we can rule out an external interference.
Regards,
Tobias