Hello GrowHero, I just finished my grow and I had the same problem I could also not join any contest. But when I did post more information on the weeks from my diary then my diary got a smiley face and when you diary got a smiley face then you can join contests like picture of the month.
Before you post a week you can see a few boxes with teks and a color it says something like pictures, update comment, grow information. When they are green then you get a smiley face. You can also edit the weeks that you've got already and just add some information and you will get a smiley face and you will be able to join the contests! Goodluck and have a nice grow! 💚