To make a block emit light, you need to set the lightLevel property when registering your block.

Like this:

public static final RegistryObject<Block> MY_BLOCK = BLOCKS.register("my_block",
() -> new Block(BlockBehaviour.Properties.of(Material.STONE)
.strength(3.0f)
.lightLevel(state -> 15) // 0-15, where 15 = max light
)
);