Nothing to do with holding a TK card. If you're using Miles&Bonus (Aegean) to upgrade, you're playing by the M&B rules. And according to them, it's usually only the 2-4 highest fares, that are upgradeable.
There actually was a TK promotion a while ago, where you'd get the B class for something like 800 EUR, so that would have been a great opportunity to try the upgrade via M&B... But in 99,9% of the cases it should be far more worth it to just issue your ticket in I, so a straight-away award ticket, instead of purchasing Eco and upgrading.
btw Here's the upgrade table: (miles for one-way flights)

Applies to all Star Alliance airlines

And here's the T&Cs, where you can check out every upgradeable booking class of all Star Alliance airlines with Aegean's miles. (just scroll a little bit to the corresponding part within Chapter 13)