Can't add search engine

[DaveScotts]

My default search engine changed to startpaage, NOT AT MY REQUEST.  I have deleted startpage from the search engine options, but I can find no way to add duckduckgo to the options.  I haave gone to settings:search engine.  The pull down for default search engine shows only Bing, Yahoo! and Google.  I have gone to Settings:Search Engine:Manage Search Engines but I can find no wayh to add a search engine on that page.  Only the same three choices, Bing, Google and Yahoo.

[nukecad]

There appears to be a Startpage hijack campaign/attack ongoing, we have had a few users report the same thing happening.
https://www.malwarebytes.com/blog/detections/hijack-startpage

I'd try first using AdwCleaner, it's free from MalwareBytes and can remove such browser hijacks.
https://www.malwarebytes.com/adwcleaner

If ADW does't clear it for you then I'd step it up the the Free (or paid) version of Malwarebytes itself and run a threat scan with that.
A scan with Malwarebytes Free will not interfere with your usual antivirus.
(If you want to run Malwarbytes Pro  in real time alongside another AV that can also be done but may need some settings changes).
https://www.malwarebytes.com/mwb-download

Note that the two can find different things, so to be doubly sure run both ADW and a MB Threat scan.

If that still doesn't clear it from your machine then see here for advice on how to clear it manually:
https://www.pcrisk.com/removal-guides/28136-abstract-start-page-browser-hijacker
(For CCleaner Browser use the 'Chrome' advice/options).
That talks about "Abstract Start Page browser hijacker", but it sometimes calls itself "Perfect Start Page browser hijacker", and I wouldn't be surprised to see other name variants.