That’s really weird, I cannot even find the error message in frob sources nor in qtads sources, not even the word “datablock”. Valid t3 file should begin with “T3-image” and then binary data follows.
QTads is the only desktop interpreter for Mac OSX which is up to date. Other interpreters like Gargoyle won’t work for new Tads games. Frobtads (a text-only interpreter) can also be used.
We may not have an accurate quote of the error message - also it could be from Gargoyle or a random obsolete interpreter.