Your key.dat file is read-only.
You have to make sure it's not.
I assume you know where your Blockland folder is, so I can skip that.
Find your key.dat file in that folder, and right click on it.
You should see this.

Click on the highlighted option.
Now a window should pop up that looks like this.

Ignore everything that isn't in the red box.
What's in the red box needs to look exactly like yours.