You've got a lot of non-Yanfly plug-ins I see. There's no guarantee that they will work. Disable all non-yanfly plug-ins, and see if the problem still occurs. If it doesn't, then add them one by one until it happens again.
Edit: Also you got two Mog plug-ins, and those almost never work with Yanfly. Disable those.
This post I've done, I hope will be very clear for you. I also want @Jrrkein to read it so they can "fix" the problem.
First I want to say I am sorry your time has been wasted like this, and I know @Jrrkein is not as helpful as they could be with there replies. I also know it's far too confusing and messy the way it being handled on here for a proper way to give bugs and sort them out easily.
However saying all that, I also knew I tested it thoroughly so I wondering why I was finally able to get the error.
I finally found the "problem". This time I used Archeia's project file from post 901467.
What I did was use the Instant Fire skill first, followed by a normal Thunder skill that consumes a turn.
If Yanfly's default battle system is set to STB, it works fine. See below
However if I set it to DTB, which is what @Jrrkein has by default, we get this error.
I must note however I also made a couple of changes to both skills. I gave them a Warrior Type and set it to target All Enemies.
I also tested this without the Steal skill, since there was never any promise it would be compatible with that Plugin. Yet I know Instant skills are meant to be compatible.
I still got the error.
Since it's clear it is meant to requires OTB for it to work, and I am almost certain if it was set to OTB it would work, I almost dismissed this entirely.
However, we are suppose to have the feature to force OTB with Olivia's wonderful Plugin, and this was set on force when I tested it with the DTB setting.
It's your call whether that counts as a bug or not. I personally would say it is a bug as other people could think leaving the default system to DTB yet forcing OTB in the OTB setting should be enough.
Other then the changes shown in these screenshots, I did nothing else to the project. I tested it on the map itself with the event that leads to a battle encounter.
Just doing the Fire skill followed by the Thunder skill should trigger it.
In case you want to quickly test it, here is both your project folders you left us with. One with the steal skill in it, one without it.
I wanted to use DropBox, but for me it came out as a complete mess so I just zipped them using winrar and added them to RMN locker space. Let me know if that is a problem.
Huh. A forum topic just pointed out to me one MV->MZ difference I hadn't registered previously: the skill/item use message in MZ doesn't have the actor name hardcoded at the beginning like MV did, so you have more control over having stuff come before it.