What does forbid mean

Just a quess. You're using CD-R Data discs instead of CD-R Digital Audio discs for recording. I imagine the manual points that out.

In reference to the question -
" what does cera and mag on the back of the mixer mean? "
these jacks are for the input of phonograph record cartridge signals.
Phonograph cartridges are of two types -
1. "Cera"mic (ceramic) cartridges generate their signal using a ceramic crystal which provides almost 1/2 a volt of current.
2. "Mag"netic (magnetic) cartridges generate a more accurate signal than a ceramic cartridge but the magnetic cartridge generates its signal using a moving-magnet mechanism (rather than a ceramic crystal) and the magnetic cartridge generates much less current than a ceramic cartridge, so its signal needs a lot more amplification than a ceramic crystal's output. So the two jacks on the back of the mixer provide different levels of amplification for the phono cartridges, less amplification for a ceramic cartridge and more amplification for a magnetic cartridge.
Since few people play phonograph records any more it is understandable that the public has little knowledge of the differences between ceramic and magnetic cartridges. But each have their own benefits and usually ceramic cartridges are cheaper than magnetic cartridges simply because it takes a lot more sophisticated manufacturing technology to make a magnetic cartridge, though magnetics are usually preferred over ceramics because of their more accurate generation of a signal from the wiggling groove in a phonograph record. But for those who can hear, the complete sound signal from a phonograph record is much preferred over the dead and dry sampled sound from a digital CD. And the old 1/4" reel-to-reel tape recordings are the highest sound quality of all because they are direct copies of the original magnetic tape masters with no digitizing at all. The fullness of their sound is astounding compared to any digitized signal..
A mag input is down at the mic level.
A ceramic input is almost up at the line level.

OPC means the unit had trouble Optical Processing Calibration, which means the disc surface may be damaged. Using brand name media and being careful to not touch the recording surface would help.

It's not completing the finalization so it won't play in a CD player yet.

It's mostly in the manual

http://www.retrevo.com/support/Harman-Kardon-CDR-20-CD-Players-manual/id/386dj191/t/2/

OPC: Stands for Optical Processing Calibration.

When you see this message in the Information

Display for the Record DeckK, it means

that the laser transport inside the unit is making

the calibrations and adjustments needed so that

the recording process is optimized for the specific
disc in use.

When an unfinalized CD-R or CD-RW disc is

placed in the Record Deck%, the letters

OPCwill flash once the Table of Contents has

been read. This indicates that the unit is optimizing

its circuits for maximum compatibility with

the specific recordable disc in use. the number

and total running time of any tracks that have

already been recorded will appear in the

Information DisplayK, and the appropriate

indicator SPwill light to tell you if a CD-R or
CD-RW disc is in use.

To Finalize a disc, place it in the Record

Drawer%. Once the disc's Table of Contents

information has been read and the OPC process

has been completed, press the Finalize button

E@and note that the Finalize Indicator

Uwill begin to flash. Within three seconds,

press the Select button9*and note that

the Finalize IndicatorUwill stop flashing.

The Finalization process will take up to two

minutes, depending on the length of material

recorded on the disc. During the Finalization

process the Record Deck will not accept any
operating commands.

Either get a self-pre-amplified phono or an old school one connected to the PHONO section of old school electronics.

Make sure the phonograph is connected to the Phono Connectors. Hook up your CD recorder to the tape. The play connectors on the CD recorders can connect to the tape play connectors on your stereo. The record connectors on your CD recorder connect to the record outputs on the tape connectors on your stereo. You can also connect you CD recorder to the VCR connectors if you are using an audio tape as your source. All of the outputs and inputs are the same level except the phono.

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End of record switch out of adjustment or bad? Disconnected?

I'd say you have the digital input selected. The player does not see a digital signal and gives such message.
Select the analog input and you should be able to record.

Hope this helps.

What have you got hooked up? Another CD, tape outs from an amp or a record deck? No audio is a strange message to get, like you say you're using an audio cdr and that's usually what it means. What about if you press record with *no* source connected? Unit should record 20 seconds of silence then cut off.

You should use consumer cd's that are rather expensive. You probably tried to record a recordable cd-rom (a cheap computer disc). It's not possible to burn a pro-disc (computer disc) by using your Pioneer, and that's why you got that message on the screen. Refer to the user manual of your cd recorder and learn what sort of blank cd you should use.