Question: Why can you not select an external 32K crystal oscillator for CY8C21x34 in a PSoC® Designer™ project?
CY8C21x34 devices do not support external 32K crystal oscillators. This part family contains resource-limited PSoC 1 devices. PSoC Designer does not have a setting that allows you to use an external crystal with CY8C21x34 devices. CY8C29x66, CY8C27x43, CY8C24x23A, CY8C28xxx, and CY8C2xx45 are the PSoC 1 families that support external 32K crystal oscillators.
More information on how to use external 32K crystal oscillators is described in AN2027 - PSoC® 1 - 32.768 kHz External Crystal Oscillator.
If a more accurate sleep clock is required while using non-external 32 kHz PSoC 1 devices, a timer user module from an external clock via a GPIO pin would be an option.