Duplex Error When Trying to Print PDF

Its is trick..If its more then two pages print, printer grabs second page even first page in duplex unit., here its confusion. Check with simplex single print, see any paper mismatch error? or printer throwing paper in output bin after successful print?. No.. check fuser out sensor or paper out roller.
Print only two pages matter in duplex, check where paper struck? if paper our roller not grabbing paper back properly in time (slipping while grabbing back) gets jam error.
Always ensure that select proper paper size in printer.Other than this it may major issue, need technical knowledge..& possibilities are less to judge the exact problem..
Keep update us to resolve the issue.

in back of machine there is a fuser unit there are 1 or maybe 2 small green buttons or switches on it that is the envelope setting push the button or switch to put it back in normal paper mode

See page 75 of the attached user manual on how to remove a duplex paper jam. If you can't find a paper jam in the duplexer, simply remove the duplex assembly (power off the printer, then power on) and see if that resolves your issue.

(The printer will operate just fine without the duplex assy. installed)
Okidata C9650 User Manual

If the error light is flashing: Check the front door and the fuser door to be sure that they are closed. Also check for any jam including the duplex section. Make sure there is not a sensor flag stuck in the down position. This can indicate memory full, print overrun, size error DX or duplex disabled.

If the error light is not flashing: it can mean there is no paper in tray 1 or the machine may be asking for you to put paper in the multi purpose tray.

Is this the only light that is on?

I've worked on these printers for years and I never heard of a "duplex transfer belt".
Sounds like you broke a wire or missed plugging in a connector.
Probably a bad Duplex Feed Drive assembly or bad DC Controller.

HP P4515n is not a duplex printer by default. It is a network printer. ('n' suggests it is a network printer. Duplex printers have 'd' in the model number).
If you want to print both sides, manually feed sheet by sheet. Or else you can do the following:
If you are using, say MS Word, print only the odd pages (Print the normal way. No need to manually feed paper). Then arrange the printed papers in the proper order and keep the printed side as back side. Insert the set of papers in the printer and print only the even pages.
First print a document of about 6 pages to get the hang of it.

The duplex unit is an OPTIONAL component on that printer.

The duplex is on the right hand side of the copier (external unit from the main print engine). You firstly need to check for a jam inside the unit, open the unit away from the copier and there will be a blue tab to access further beyond the black plates. Also make sure the unit is seated correctly and plugged in (Turn off unplug and plug back in, this will also reboot the machine). This is customer recoverable error so the reboot should fix it

message error 900.00 service rip software on lexmark c530dn

Can you save that one document as a pdf and print it?