1) I've noticed signs/nor the cannons work when you set 'isSignRequired: false' to true. This only seems to affect 1.15 versions of cannons/spigot. Didn't have this issue on my 1.12.2 server.

In the video i'm editing the default classics cannon:
https://www.youtube.com/watch?v=c1rhhiQNjRE

2) SpawnOnExplosion seems to have changed for the 1.15 version incomparison to 1.12.2. I had a projectile with pen 3 that would pierce through blocks and spawn cobwebs.
EDIT: penetration seems to be broken or not working how it did in 1.12.2. Even with my normal cannon ball shots... the damage has changed.

These projectiles have the same settings, just in different versions:
penetration 1, explosion power 4.
https://drive.google.com/file/d/1y8x1jGAERBmNsFqXPYZ1XX9dpUAy_BQd/view
https://drive.google.com/file/d/162iOHte1qStF4yQ6WOQQADO3ZW8L7kqm/view

3) I also get this console error every time i load a projectile.

1. I can confirm that, seems to be a problem with the material types of signs
2. Penetration seems to be working for me on my Spigot server, with 3 I get a penetration of 2-3 blocks. With explosionpower of 2.0 I get a radius of 2 in wool. If I set the penetration to 10 it punches a hole throw the wall and explodes on the other side.
3. You are using ENTITY_IRONGOLEM_ATTACK which is missing an underscore. Try to replace it with ENTITY_IRON_GOLEM_ATTACK
   https://hub.spigotmc.org/javadocs/spigot/org/bukkit/Sound.html

@DerPavlov
Ah thanks! I did another test with a projectile that is pen 6.
https://drive.google.com/file/d/1qgLYRsB1Xf4_wWWPcTBZ6LrJTH82gmSa/view

Penetration seems to only punch 2-3 blocks deep. Did you change how penetration works in 1.15 any chance? Like perhaps adding that range pen feature i suggests, if so then maybe its better removing it.

EDIT: i tested with penetration 10 and it does seem to dig in deeper, but its quite varied in performance. it makes balancing my shots quite hard. like sometimes it goes 3 deep, other times 5-6 blocks deep.

Like this shot was pen 5 and meant to spawn blocks ( i shot it at a diagonal angle)

Judging by the picture I think what you see is just the explosion and not penetration. Otherwise it would be a clean 1x1 block hole with a length of 10 blocks.
Additionally, there is a certain randomness and a scaling factor for the speed. Since your target is pretty far away it might be already too slow.

You can enable the debug feature to display these values.

@DerPavlov
Thanks for the debug suggestion it shows interesting information. Did some testing against a thick wall that was only 20 blocks away. I tested my my penetration 3 shot which has no explode damage but a spawnonexplosion block spawn. Also made it velocity 4 so it was fast. Below are the results i got.

1.15.1 + latest cannons version...

^^ that 'randomness' bit might be making the penetration random each time? did you make that penetration range addition because it might be worth removing if its causing this issue.

1.12.2 showed this...

EDIT: I've been told it's likely to do with the new 1.14 explosion calculation changes. is there anyway you could get penetration to work like how it did in 1.12? or in a way that makes it more predictable. It seems to be quite variable when it comes to pen etc and no longer matches up with 1-10 pen numbers.

This test i did was using velocity 3 and pen 1. but damage no blocks (and had 0 pen on debugg).

Velocity also on the debug reports seems to vary, some scatter type bullets dont all damage. and i get this report which states my velocity was too slow at 2.3 despite it being set to 3 and my target is only 15 blocks away.

@DerPavlov Whats your opinion on the above, is penetration fixable or is this 1.14 changes? It makes penetration very random despite turning off the 'speed effects power (kinetic energy) setting you have). Thanks.