The documentation in section 16.1.2. Basic Text Elements says
<p:font> Description The font tag defines the default font to be used for all text inside of it. Attributes name The font name, for example: COURIER, HELVETICA, TIMES-ROMAN, SYMBOL or ZAPFDINGBATS.
What is the error?
I had the same problem... I tryed to use a custom font like "Tahoma", or "Verdana" and SEAM PDF didn't accepted it.
How can I use a different font from the list in the SEAM doc?
There's no support for that. Only PDF fonts are supported. You can open a JIRA issue if you like. In generally iText has good font support, so it's probably quite feasible.