A 14 works. However 5e more ore less assume that if your class primary is a 15 or 14 at level 1, you are planing to powergame a bit more that base assumption.
For example, you want to be a gnome fighter. A breachgnome. Base Gnome gets +2 INT. The Subclasses offer +1 DEX or CON
5e assumes that you are either using the +2 INT to go Eldritch Knight or uses the +1 DEX to make a Arcane Archer.
5e does not want you to make a 15 STR Gnome Champion fighter unless you DM is planning to allow you to dip into power elsewhere (feats, multiclassing, gifts, house rules, favoritism).