Managing 100% discount code

Hello,

I am trying to create a one time use 100% discount code for a colleague to register on my site for free in order to do some testing etc.

Once I generate it successfully for a PayPal plan, the site sends me an error saying that certain parameters are not met.

Is there a way to simply create a free plan that is secure, I can use to let my friends get free access?

Also would this work for ppl that I give promo codes to?

Thanks for any help you can provide.