You already have a Split Pay account tied to that phone number. The screen is letting you sign back into the existing account instead of creating a new one — tap "Continue to Sign In" and you'll be logged into your original account.
Each phone number can only be connected to one Split Pay account, so creating a duplicate isn't possible. If you tap "Continue creating account" instead, you'll end up with two accounts and one will need to be deleted later.
If you don't remember which sign-in method you used originally:
Check your inbox for the welcome email from Split Pay — it'll indicate which method
Apple users: your account email may be a private relay address that looks like
xxxxx@privaterelay.appleid.com
If you genuinely can't get back into your original account, contact support and we'll help.