Solution
Based on the
NI 9775 Specification and the help file
NI 9775 Considerations , if you want to acquire in continuous mode, the maximum sample rate for one channel will be 4 MS/s aggregate, and 20 MS/s in record mode.
The following table shows what is the maximum rate per channel per mode:

For example, if you want to reach 5 MS/s or more, you will need to use the record mode.