IF AND Formula

try this

IF(

[Cybersecurity] = true and IsNull([Engagement Begins), "Upcoming",

[Cybersecurity] = true

and not IsNull([Engagement Begins])

and

IsNull([Cybersecurity Discovery Engagement Ends]), "Assigned")

.....

Some notes about how Quickbase formulas work. The first is that unlike Excel, in general you don't need to nest your IFs. You just have one IF( at the beginning and you list your conditions and the result in sequence, and the formula will look for the first condition which is true.  

The IsNull function can use can be used on (almost) any field type but it cannot be used to check for a text field being blank.

To test if a text field is blank you would use IF(Trim([my text field])="",

The Trim is just in case some one enters some spaces into a text field.