@BECQ , a couple of troubleshoots:
1. have you enabled attachments in AGOL yet?

2. As @PeterMacKenzie2 said, you can assign attachments in the attribute table by RMC'ing. You can also open the Attributes panel from the Edit menu in Pro...

...and then add attachments there:

3. But if you're looking to do this in AGOL, you can add attachments via the Data tab from the Item Description menu:

4. Or if in a web map, you can edit features or add features and add attachments from the editing popup:

David
..Maps with no limits..