Lightning spawns block

i dont think that is possible, an alternative though could be, during a thunderstorm a mob will have a chance to spawn and it instantly dies and on mob death strike lighting in that location and place block

Simple make a never ending procedure that runs off world tick or etc
That would execute the following command as the Console itself..
 

Execute @e[type=lightning_bolt] ~ ~ ~ /setblock ~ ~ ~ stone
replace stone with Whatever you want.
For instance, amod:blockthing

1.13 and above would be different,
I'm a 1.12.2 pleb ;P

For an effect Around an entity, block or etc
You would do the same but with the following little variation And running off Their tick instead of the world,
NOTE - you must maintain this running via the Console not the player - entity or etc! Otherwise you may have Permission conflicts.

Execute @e[type=lightning_bolt,r=10] ~ ~ ~ /setblock ~ ~ ~ stone
replace stone with Whatever you want.
For instance, amod:blockthing
Replace the "10" after the r= with whatever Block Range you want.

hope this helped.

lightning is an entity, so you can create a procedure element, and in the first green block there's an event trigger pulldown, choose "Entity joins the world". and then use IF/DO blocks to check if the current entity is subtype of EntityLightningBolt

just make a procedure that runs this command every tick

/execute at @e[type=lightning_bolt] run setblock ~ ~ ~ <mods namespace>:<block you want to place>

Oh, what finally got it to work?