Grow action help

[Arnetto]

I am struggling to understand why the first Grow action, gain Strength-1 workers, is not available to me when I play a Strength-3 worker there. It lets me use Pollen for the frame action but the gain worker button is grayed out. What am I doing wrong? Please help, thank you
https://boardgamearena.com/10/apiary?table=790968470

[jordijansen]

You can only have a total of 4 workers. Since 3 are on the board already, and the last one is in your active pool. You can't gain a 5th.