HaloMods "Classic" Read-only Archive

I keep getting this error in tool everytime i try to build a jms file. I never had this problem before, so i have no idea wtf is going on. at first i thought it was b/c the model was high poly, but then i tried a low poly 1 and got the same error. I blocked out the name of the weapon so no one would steal my ideas. Any ideas on how to fix this?

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