How do I make mod's blocks burn?

You can by editing the code. You need to add this piece of code to your block constructor (where you see setResistance, setLightLevel etc.)

Blocks.FIRE.setFireInfo(this, 5, 20);

5 is how fast the block catches fire, 20 is how fast the block burns.
There's also a ticket open for this in the tracker, you might want to upvote it if you want burnable blocks in MCreator.

What did the error say? Try pressing ctrl+w after adding the line, then save and recompile.

C:\Users\PC\MCreatorWorkspaces\levapap_modification\build\sources\main\java\net\mcreator\levapap_modification\MCreatorWandBulletHitsBlock.java:227: error: non-static variable this cannot be referenced from a static context
            Blocks.FIRE.setFireInfo(this, 5, 20);
                                    ^
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 29s
 

See the comment below…

The line of code doesn't set the block on fire, it makes the block flammable. It's a property of the block, not an action. If you edit the code of a block, you should see something like this:

public BlockCustom() {
	super(Material.PLANTS);
	setRegistryName("lightbulb_lamp");
	setUnlocalizedName("lightbulb_lamp");
	setSoundType(SoundType.PLANT);
	setHarvestLevel("axe", -1);
	setHardness(0F);
	setResistance(0.5F);
	setLightLevel(0.95F);
	setLightOpacity(0);
	setCreativeTab(CreativeTabs.DECORATIONS);
}

This is where the line of code goes.

If you want to set a block on fire with procedures:
1. Make the block flammable by adding the line inside the code of the block;
2. In the procedure, you could use the "Place block at x y z" block to place a fire block above the flammable block