I asked for the screenshot of the skill/script call in post #2 above...
It might be a compatibility problem with other scripts, but it could be something else - but links to the other scripts you're using might be helpfull.
I can't see anything directly wrong, but I haven't worked with that reinforcement script myself.
There are four possible sources for that error that I can imagine at the moment, and you'll have to test a few things to check those cases.
1) The current error might be from a different script, having nothing to do with summon monster.
Please install
http://himeworks.wordpress.com/2013/06/09/custom-main-full-error-backtrace/
and check the output for the script name that really caused the error
2) Tsukihime's script might require to be called from the troop event instead of the common events
Just test if the add-command works when you place it in a troop event instead of a common event - all examples on Tsukihime's blog use troop events, and it might be that a specific data access is not possible from the common events (that's just guessing, I'm not that deep into the RGSS engine to know if there is a difference)
3) The problem might be in the enemy or the attack pattern
Please provide a screenshot of the enemy and the attack condition for the use of that summoning skill
4) Compatibility problem with a different script
That's where the list of other scripts would come in handy - if another script replaces troop behaviour, it can conflict with Tsukihime's changes - even if that other script has nothing to do with summoning, but changes troop display or adds battle conditions or whatever.
Test if the skill and script work in a new project where no other script is added - if yes, disable the scripts one after another in a copy of your main project, until you've found the conflicting script.