That's okay! You absolutely can still subscribe!
If you got a message that your email address is in use, most likely you had a TouringPlans.com account in the past. You may have used a different product (perhaps you had a Disneyland subscription but now you want Walt Disney World), or you may have subscribed via our Lines app. But you can (and should) use a single TouringPlans.com account for all of that.
What you need to do is login to your old account at: https://touringplans.com/login.
There are "Forgot Password?" and "Forgot Username?" links on that page if you have forgotten either or both of those items.
After logging in, you can go to https://touringplans.com/join to subscribe.