Heyo! The achievement only drops upon earning one of the original seals introduced in Forsaken. This from [url=https://www.bungie.net/en/Help/Article/48484]Bungie’s Help Article:[/url] [quote]The Seal the Deal Achievement/Trophy will only unlock if a player owns Forsaken and one of the following Forsaken seals is earned: Wayfarer Dredgen Unbroken Chronicler Cursebreaker Rivensbane[/quote] Hope this helps!
Seal the Deal will only unlock when you complete one of the original Season 4 seals. You an find a list of them in this help article: https://www.bungie.net/en/Help/Article/48484