Here's a handy dandy possible error list!!!
(1) Find if you have soil.
(2) build farm plot on soil
(3) Verify whether it's subterranean or above ground
(4) Make sure you have seeds for a plant that is available to underground or above ground.(depending on step 3)
(5)Make sure the letters aren't reddened (ex. = Rope Reed)
(6)Make sure you have an idle person (ex. = Human, Dwarf, Elf, ect.) and put the job "Farming (fields)" as white. (ex. = Farming (fields))
(7)Be sure they are marked as "no job"
(8)Create or verify a path from the person with the farming (fields) job and the farming plot.
(9)Be sure the person does not have any vital health conditions (ex. = Unconscious, winded, or any body parts colored red (ex. in the ex. = Lungs))
(10)Be sure the person has access to the seeds
(11) (this goes along with 9) Moods can also effect actions. (ex. Possessed, Fey, ect.)
(12) see the person is alive
(13) if it is outside press "o" and see if peoples are allowed outside.
(14) See that the person with the job is not a soldier, guard, or royal guard.
(15) Check if the plant can be planted. (some plants are harvestable only)
If it still doesn't work, Give it a few minutes... After that, try giving the "farming (fields)" job to another dwarf. If it still doesn't work, try to remember if you've done any modding lately, you may have accidentally changed something vital in the txt. If it STILL doesn't work, I have no idea what to do.