Question-and-Answer Resource for the Building Energy Modeling Community
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This isn't a full answer, but I've found it sometimes can be helpful when encountering an unfamiliar error to search the EnergyPlus source code using a bit of the error string to see what comes up. In you case, this error only shows up once, when (and this is kind of a stretch, since I don't know C++ that well) the plant loop index is zero.
I would double-check that you've connected your SwimmingPool objects correctly to the proper demand branch(es) of your loop. If you're still getting the error, you might want to upload your idf.
