Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32c0/STM32C071/TIM2/TIM2_AF1#0x0
ETRSEL=B_0x0
TIM2 alternate function option register 1
ETR source selection
0 (B_0x0): ETR legacy mode
3 (B_0x3): LSE
4 (B_0x4): MCO
5 (B_0x5): MCO2
https://github.com/modm-io/cmsis-svd-stm32