As I play the game, it won't let me plant grain right away, either.
But I can plant the hemp plant.
The hemp plant looks strange, sort of like a thin green spike with some leafy bits about the center. It's about person-height.
It took lots of hacking at it, but eventually I got the "light emanating from the heavens" thing and there was a single hemp leaf lying on the ground.
When I took that leaf and planted it--
(1) put it in my hand, in the rucksack view
(2) closed that view
(3) in the regular view I aimed at the ground, then pushed the "e" key.
(4) it didn't really look like anything happened--the new green hemp plant is the same color as much of the grass in the game.
(5) however, if I tried to plant a second plant, it would pop up an error (red type) message telling me that it needed more room.
(6) so I backed up a bit (or just turned around) and hit the "e" key again.
(7) when the plant grows (in a couple of game days) I then could wail on the plant again and it eventually yielded 3 of the hemp leaves.
(8) there doesn't seem to be a use for the hemp if you don't want to make endless marijuana joints. Pity, since hemp is so useful for so many things. It makes great rope, and even can be used to make hats and clothes
(9) But it will improve your planting skill until you can plant grain. (you'll know when you get the heavenly light and message "your skill improved".
I hope this helps. I struggled for quite a bit before discovering exactly how to do this. Just be patient, and continue to plant and harvest, plant again and harvest again, (rinse and repeat . . .) It appears that your planting skill will rise as you plant more things--even lots of the same thing.
This ended up being a tremendous "game enhancement" for me! After my planting skill had progressed a bit, I could plant berries, which grew into berry bushes! Now, fresh berries for breakfast every day!
I also planted some grapes I found in a "flotsam box" (floating box in the ocean, near the beach. Made a wonderful little vineyard!
I created an amazing landscape of tree-lined walks and scenic points by planting trees and bushes. Even though you can't control which kind of tree will come up from any particular branch (it's random) you can cut down and replant any trees until you get the kind you want in that spot.
This also helped me to avoid turning this island paradise into a barren piece of rock by cutting down all of the trees to build stuff. Now I can plant back new trees whenever I harvest them.
Hope this helps you through a somewhat frustrating experience!