Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32l0/STM32L053/TIM21/OR#0x0
TIM21 option register
Timer21 ETR remap
Timer21 TI1
Timer21 TI2
https://github.com/modm-io/cmsis-svd-stm32