Easy mistake to make, just take a look at how much population you have and ensure that you have enough to cover an award and you'll be fine
Incase you are unsure, open your training screen, click on the unit and see how many population is required per unit and do the maths as to what the award is to how many population you have to have...you can always add a level or two to your farm part way through an award being active to give you more population if the build time of the farm level is less than the award training time