Unfortunately, that device does not have any digital timing engines, so you cannot hardware-time digital acquisition. The higher-end M Series (62xx) devices support correlated DIO timing, where you use another clock (external clock, internal counter, analog timing engine, etc.) to clock the digital subsystem, but the cheaper 621x devices do not support this feature. Here is some documentation that explains this:
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P7gjSAC&l=en-US
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000kHNdSAM&l=en-US
Depending on what you actually want to capture (PWM, for example), you may be able to use counters to achieve what you want. See Chapter 8 of the User Manual:
https://www.ni.com/pdf/manuals/371931f.pdf
