The sentence "I understand your frustration" is correct and can be used in written English. You can use this phrase when someone is experiencing frustration, as a way of expressing empathy. For example, you could say "I understand your frustration that the project is taking longer than expected. Let me see what I can do to speed things up.".
I understand your frustration — really, I do.
DAVID REMNICK: I understand your frustration.
I understand your frustration, but don't lead with your checkbook (or first-class mail, either).
"I understand your frustration because you're responsible for your entire business," Ms. Johnson said.
"I can understand your frustration," Keller said.
I can understand your frustration, Rhona, if you're just taking baby steps all the time, from stepping stone to stepping stone.
"I fully understand your frustration," the chief of staff of recruiting command responded in the magazine, adding that a new lanyard was now available that would be "more widely accepted within the high schools".
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com