Final answer:
The error in the sentence is the use of "phased" instead of "fazed." "Fazed" is the correct term when expressing that one is not disturbed or bothered by something, such as bad weather.
Step-by-step explanation:
The error in the sentence "What an amazing day! It's supposed to rain again tomorrow, we're not phased by a little bad weather." is the use of the word "phased" instead of "fazed". The correct sentence should be: "What an amazing day! It's supposed to rain again tomorrow, we're not fazed by a little bad weather." The word "fazed" means to be disturbed or disconcerted, which fits the context of the sentence appropriately, whereas "phased" refers to stages or steps in a process, which is not the intended meaning here.