New member need help

Hello!
I have a question. If I play a creature in instant speed (either by spawning tokens in instant speed or just a creature with flash) I can't seem to use it as a blocker. I have to play it as my computer opponent's main phase ends (something called Before Combat phase?), so my opponent knows that I have blockers as they assign attackers. This is not the optimal play in a normal game irl, is it? I've read the MtG rules and I don't see anything that prohibits a player from blocking with a creature spawned at instant speed in the declare blockers step. I have attached a picture to show the situation and I cannot block, it won't let me.
Is Forge wrong, or is my idea wrong?
Thanks for your help
I have a question. If I play a creature in instant speed (either by spawning tokens in instant speed or just a creature with flash) I can't seem to use it as a blocker. I have to play it as my computer opponent's main phase ends (something called Before Combat phase?), so my opponent knows that I have blockers as they assign attackers. This is not the optimal play in a normal game irl, is it? I've read the MtG rules and I don't see anything that prohibits a player from blocking with a creature spawned at instant speed in the declare blockers step. I have attached a picture to show the situation and I cannot block, it won't let me.
Is Forge wrong, or is my idea wrong?
Thanks for your help