Hi @Anonymous,
Welcome to the community! 
Here is the link to the original notice which I posted about a month ago.
"For the regular Autodesk Seek users out there, take note that Autodesk Seek will be transferred to BIMobject.
Read the Press Release HERE and visit BIMobject HERE
"BIMobject AB (“BIMobject”) announces today that it has signed an agreement with Autodesk regarding transfer of the operations of the Autodesk® Seek business to BIMobject. As part of this arrangement, BIMobject will assume the operational responsibility of providing Autodesk® Seek customers with high quality building information modeling (BIM) content services on the BIMobject Cloud."

Herewith a link to Doors available on BIMobject.
Herewith a link to Doors available on RevitCity.
Default location for OOTB (out of the box) Revit Content Library Doors:
Revit 2016 : C:\ProgramData\Autodesk\RVT 2016\Libraries\US Imperial\Doors (Choose between Imperial or Metric)
Revit 2017 : C:\ProgramData\Autodesk\RVT 2017\Libraries\US Imperial\Doors (Choose between Imperial or Metric)
Also remember that as @barthbradley mentioned, load new families by going to the Insert Tab >> Load Family Library >> Load Family. Sometimes people/students might toggle the load family from e.g. "Windows Type Properties" and then try and load Door Families, this will then prompt them with an error message.
I hope this helps! 
Rudi Roux
MSc | Digital Engineering Manager
LinkedIn
Revit Mechanical & Electrical Systems 2018 Certified Professional | Revit MEP & Architecture 2015 Certified Professional
AutoCAD 2015 Certified Professional | Autodesk Building Performance Analysis (BPA) Certificate
If this post resolved your issue, kindly Accept as the Solution below. Kudos are always welcome ⇘