What is your application id and the location_id being used? I can take a look at our logs. However, I want to call out that the Transactions API is currently deprecated and is planned to be fully retired (no longer guaranteed to work) in September of 2021. You should migrate to the Payments API as soon as possible.
Thanks for that information! I took a look at our logs and found that we’re not actually receiving the verification token, which is why you’re receiving that error. Looking at the code you provided above, I believe you need to change verificationToken to verification_token (underscore vs camelCase).
Also, I would like to note that we do have a new PHP SDK that I strongly recommend you to migrate to. While the old SDK will not stop working, it is technically deprecated and is no longer receiving new features or updates. Please let me know if you have additional questions or concerns!