If you forget to add sugar, the amount of alcohol (% ABV) will be lower than expected since there is less sugar for the yeast to ferment and turn into alcohol.
While the beer itself will be fine, body and mouthfeel will be different - the beer will most likely be drier and have less body than if the sugar was included.
To help, you can:
- Dilute the sugar in a jug with 1 L (1.1 US qt) of water
- Cover this with tin foil
- Allow to cool to room temperature
- Then add this litre of sugar water to the fermenter during active fermentation.
The beer will not be perfect but will be closer than not adding the sugar at all.