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This error comes as a bit odd to me when I tried to export a model just now for TF2. The weird thing is I have exported MANY models into TF2 already but maybe after the large recent update that just happend something was changed idk?
I'm sure I've seen that error if there is a Steam Update and I tried to compile something without first running Source SDK in Steam at least once. Let me know if that solves your problem.
There we go, that's all I had to do lol!
Thanks for the fast response :)