Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32l5/STM32L552/FDCAN1/FDCAN_RXF1A#0x0
FDCAN Rx FIFO 1 Acknowledge Register
Rx FIFO 1 Acknowledge Index
https://github.com/modm-io/cmsis-svd-stm32