When I would like to say - it's easier yhen i thought ---- can it be just --- E piu facile di credevo ??
If you actually used the word 'then' in your translation, that's the problem. You wanted 'than.'
I would think it is incorrect because you have used a conditional clause.
In the exercise going from English to Italian, the "correct" answer is given as È più facile di quanto pensassi - in subjunctive tense, before we get to the subjunctive module. And there is no che in that sentence. One user commented that Duo accepted di quanto ho pensato.
Obviously, an error of some sort is happening with this particular sentence. Very confusing.