Trying to plant seeds on certain farmland blocks in 1.16.1 won't work. You can make the dirt into farmland and it will turn into wet farmland, but you can't plant the seeds. I'm not sure if this is a world problem but it could also be those specific coordinates causing the problem.

The video attached shows the problem. You can't see me left clicking, but it won't work. I left my debug information at the start of the video. You can also see the other wheat plants around me; I was able to plant those seeds just fine. It appears to be those blocks. I haven't tested the rest of the empty farmland yet, but this is at least the start of the problem.

Seed: -1633050648366710529