Can't complete a battle with new party member

Hello all, I'm a first time user of RPG Maker and I'm having something of a problem. I really love this program and have wanted to do something like this for a long time, the only issue I seem to come up against is my inability to figure out how to add a party member without it causing an error. I'm able to make an event to have my main character talk to an NPC and have him join the party, but then when they finish fighting a battle together, it says:

Script 'Game_Party' line 133:ArgumentError occurred.

malformed format sting - %'

As I mentioned before, I'm pretty new to this but I checked my Script Editor and line 133 of Game_Party says:

return sprintf(Vocab:partyName, leader.name)

When I start an entirely new game file and check the Script Editor, it says that too, so I figured that's the default setting. I'm just not sure what I'm supposed to do to get rid of the error, or why the error is occurring in the first place. Any help would be greatly appreciated. Thank you in advance.

Do you have any scripts installed (other than the standard ones that are in an empty/new project)?

Could you provide some screenshots of the event for the doing the battle, and also the database screens for the actors that you are using?

One of those might shed some more light on what is happening.

Thanks for your help. I realized I had been messing around in the Vocab section to change the victory and battle messages. There was a typo on line 26 where the PartyName is written and I must've accidentally deleted the %s on that line. Thanks to your message I went back and rechecked it, sure enough it was just a mistake I made earlier this week that I didn't notice. Thanks again!