Can I return the 'Tech 21 FlexPro Case' if it fails to protect screen protector on my iPhone?

Generally, no unless within the initial 14 day return period. Warranties on third party products are serviced by the manufacturer. Apple does not warranty third party products, their manufacturer does.

Now, with all that said, you’re obviously disappointed in the performance of a product. I’d return to the Apple Store you purchased it at and speak with a store manager and seek their opinion and advice.

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