Sondra wrote...
I'm working on a project for my class and they want me to "test whether
there is a value in a cell in order to sum it up"
....
. . . They want us to do ISTEST, but I can't find any
information on this. *I am beginning to think its a typo.
....

Best course of action would be to ask the instructor whether ISTEST is
a typo or whether it's a placeholder for a function which Excel does
provide.

Your instructor may want you to check the value type, but in the real
world if *expected* results were either text or number and all you
need to do sum it, you wouldn't check value types.