To ensure your Place Exchange credentials are correct, simply replace the Org_ID and Ad_UnitID and enter the URL in a browser:
https://api.placeexchange.com/v3/orgs/Org_ID/adunits/Ad_UnitIT/adrequests?format=vast4
If all is correct, the page should load a content that looks like:
<VAST xmlns="http://www.iab.com/VAST" version="4.0">
<Ad id="31d23e9b-aoab-ioc3-zkpd3-72237923237">
<InLine>
...
</InLine>
</Ad>
</VAST>