My et103c will not manually or electronically turn off my pool pump.

Copy following link to identify timer and download manual:
http://waterheatertimer.org/Intermatic-timers-and-manuals.html#ET-series
Unknown if this is new installation, or old installation that stopped working.
Check battery. The timer requires a battery. Located below the buttons.
ET103C digital electronic timer is discontinued but some folks still sell on ebay... selling possibly used timer. Timer has 1 year limited warranty from date of manufacture.
Timer has error code messages... if you receive no error code, then battery could be problem.
Line noise can cause timer not to function, such as fluorescent lights on same circuit etc.
ET103C clock mechanism is 120Volt 60 Hz. If pump and circuit are 208-240Volt, or 277V etc then this timer is not best choice unless neutral wire is available..
Add a comment and describe situation in more detail.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7