The fills don't work – I want to add oaks, but it adds birches instead :mad:
Is there any way to fix this, or will I have to add birches? I'm sure I selected oaks.

If you really want oaks, why not just add them to the map manually by selecting the desired objects from the list? You don't have to use fills.

Added 4 minutes later

Hello everyone. I have a problem. When my army is larger than the computer's (in the early days of the game, in battles with neutrals, and there are legions of them), the hero loses his army and continues the game with an empty Castle and a couple of golems. The computer stays in the Castle and waits (even if I'm on the other side of the map).
How can I make it more aggressive? Thank you in advance.

Perhaps the function SetAIPlayerAttractor( sObjectName, nPlayerID, nPriority ), or SetAIHeroAttractor( sObjectName, sHeroName, nPriority ) will help.
The first is for the AI player in general, the second is for a specific hero. sObjectName is the name of the object that the AI should hunt. This object can be a hero. nPriority is the priority of the target. By setting it to two, the AI will be eager to kill or capture the target, risking itself (at least that's what the function description says).
There may be another option. You can open the map in the editor, find the player properties section in the map properties tree, and change the behavior of the AI you want from random to Warrior. Maybe after that it will become more aggressive.