"Add Existing Product" Button under Quotation Manager Is Not Working

[rsjpxEC]

Greetings,

I'm using Magento 1.8.1.0 and I'm unable to "Add Existing Product" or "Add Custom Product" under Quotation Manager.  I'm not sure what I'm missing.

Also, a pop-up comes up when attempting to go into the Quotation Manager:

I pasted the error of what comes out when I add a product.  Maybe someone can shed some light on this.

https://www.entercomputers.com/errors/report.php?id=243459197239&skin=default