No, poster is not a publication.

Do Poster Presentations count as publications eras?

ERAS includes a sometimes confusing list of options for listing publications and presentations: Peer Reviewed Journal Articles/Abstracts. Peer Reviewed Journal Articles/Abstracts (Other than Published) ... Poster Presentation.

Does a poster at a conference count as a publication?

"in a reputable journal" - In computer science, posters are often submitted together with a short paper, which is included in the conference proceedings. This would count as a (small) publication as well.