As InVision allows to grab embed code for mobile prototypes only, make sure you have your prototype in a mobile format and not a desktop or freeform.
To embed your InVision prototype do the following steps:
1. Make sure you have your prototype open and click that dotted MORE icon next to ASSETS and select Embed Prototype.
2. Switch to IFRAME tab and copy the code.
3. Paste the copied code to one of the Embed Code boxes in your map or persona at UXPressia and it will render in a few seconds like that:
Read more on embedding screens and prototypes the the official InVision help center: