I'm sorry to hear about your continued difficulties.
The category you were using when you started this thread (# 174049) is still a valid category in which to list your item, but eBay just does not allow the use of the "Good 'Til Canceled" duration in that category any longer.
Therefore, if you attempt to post the listing in category # 174049 but select a different duration (such as “30 days”), I believe you will be able to post it successfully.
I believe the eBay UK site map you are referring to must be outdated or otherwise incorrect because category # 74800 is not currently a valid category for car parts. If you search for that category on the Sell your Item form directly on the eBay UK site, you should notice that no categories related to car parts are returned.
If you need any further assistance with this matter, please follow up with us through your open support case on the subject. I just added this information, along with a few other details, to your case as well.