I have a straight talk iPhone and I need to unlock it to use on Verizon.

You need to talk to Straight Talk and have them unlock it. Only the carrier an iPhone is locked to can unlock it.

Terms and Conditions | Straight Talk Wireless

VII. STRAIGHT TALK UNLOCKING POLICY

For Customers with Straight Talk Phones Capable of Being Unlocked: Straight Talk will provide an unlocking code to customers who request unlocking, provided they meet the following conditions:

(a) Customers must request phone unlocking. Straight Talk will unlock phones of current and former Straight Talk customers without charge. Non-former customers may request unlocking, but Straight Talk may charge a reasonable fee.

(b) The Straight Talk phone must have been activated on Straight Talk Service for no fewer than 12 months with Service Plans redeemed in no fewer than 12 months.

(c) Customers must possess a Straight Talk phone that is not reported stolen, lost, or associated with fraudulent activity.

(d) If a customer in good standing requesting phone unlocking is deployed as military personnel, upon provision of deployment papers, Straight Talk will honor that customer's unlocking request.

Note that iPhone's do not use unlock codes, however Straight Talk must still request the unlock from Apple.

More details here:

How to unlock your iPhone for use with a different carrier - Apple Support

You are going to have to ask Straight Talk to unlock it for you. Since they are holding the lock, they are the only one that can unlock it.

ONLY the carrier to which it is locked can unlock it.

Contact that carrier and ask if they offer unlocking and if you qualify.