Translation:Adamo could not attend the conference in Poland.
As in English, being at an even is not quite the same as attending it:
- La konferenco okazis ĉe hotelo. La hotelestro estis ĉe la konferenco, sed li ne ĉeestis ĝin.
- (The conference was held at a hotel. The hotel manager was at the conference, but he did not attend it.)
Because it is the past tense "povis" not the hypothetical "povus." Basically, the sentence is saying that the conference already happened and Adamo couldn't make it. Not that, if it were to happen, he wouldn't be able to make it.