Hi Fan,
You are using what is called “Triple Generator”, which is included in ADCpro as a generic EVM signal emulator, but it is not the ADS8881 software.
As it is specified in page 9 of the ADS8881EVM user guide, before using the ADS8881EVM with ADCpro, you need to install the ADS8881 software found at the product folder:
www.ti.com/tool/ads8881evm-pdk
Then, if the EVM is properly powered up and configured, you should be able to change sampling rate as shown below:
Regards,
Rafael