Yes, it is free. You can use any CC version with the same subscription.
Thanks for your reply，
My ai cc 2015 is a enterprise edition，it belong to the company I working for，but when I update to 2017，there have been some problems. The photoshop cc 2017 can use, the ai cc2017 become a trial version，and it can’t be activated.
Make sure you are logged into the cloud with you companies email address, and not your personal email address you may use for the forums.
Try clicking buy now, follow the prompts as far as you can, and see if that will clear this up.
Failing everything else, I am afraid you will have to ask Adobe, through a chat (just click the Still need help button straight away),
or through Adobe phone Support, see the different workday hours depending on country),
You need to check with your IT department. If they licensed Creative Cloud for you, then you can download all apps.
Right now you only have Photoshop and Camera Raw, so they may have gotten you either the Photographers plan or the one software plan, and neither one includes Illustrator or any other application. That is probably why you are getting this message about Illustrator.
If you go to Options > Manage Accounts and know your log-in, you may be able to find out what Plan & Products you have.
I strongly suspect they did not purchase the full Creative Cloud license for you, but you will need to confirm it with IT, looking at your account, or talking to Adobe.