Why does my printer jam repeatedly?

It's a 6-7 year old printer.  Don't know how often you use it, but the pickup rollers could need cleaning or replacing.  Honestly, at that age, I wouldn't consider it economical to try to repair if you can't do it easily yourself.  Just get a new printer, it's probably cheaper.

I couldn't find any direct instructions for that model.  This may help: https://smallbusiness.chron.com/clean-hp-printer-rollers-55269.html

Lee's almost certainly right about this one.  The pickup rollers which grab the paper are made out of a soft polymer which -- when new -- is very grabby and picks reliablyl.  As time goes on, oxygen and ozone harden those rollers and the surface fails to grab paper.

There used to be a product used for reconditioning typewriter platens which would add a thin layer of rubber to the surface and this would often solve the problem for a few months.

Cleaning the rollers with isopropyl is often recommended but usually makes the situation worse.  Isopropyl eats into the polymer and accelerates the hardening.

It might be possible to get that part but it's not intended to be a field-replaceable part so the actual replacement is a nightmare (I tried doing a pick roller replacement once) and the way printers are made these days, replacement is likely to disalign something critical so the printer won't work right afterward.

What I'd do:  Go to the local computer recycler, tell them you want a newer printer, and pay no more than $20 for it.  And avoid HP if you're buying a new printer as the ink costs a fortune.  Epson makes printers with one-liter ink tanks that seldom need refilling.

Check your paper is dry and fans easily (old stocks can get stuck together).
If this is new and happens with a reliable paper stock the pick up assembly (an eccentric, rubber coated cam) which rotates to pick up the top sheet either need cleaning or replacement.  It's sold as a maintenance kit by HP.

See pages 43 onwards in the service guide (attached)

(Edit: Actually in this AIO it's not as difficult as Dr Klahn suggests but YMMV!!)
Separator/pick assembly kit HP Part No. CN598-67018
hp-officejet-pro-x451-x551-repair-ma.pdf

Dr. Klahn, can you recommend a color laser or inkjet with duplexing?   Thanks.

I've never needed a printer with hardware duplexing, but I'm sure one of the other experts will be able to recommend a suitable unit.

i vouch for a lasrejet; they cost initially more, but don't need ink replacement often; print faster, etc
here a site discussing the BEST options  qua price, performance and for business
https://www.digitaltrends.com/computing/best-color-laser-printers/

btw - i have HP Laserjet PRO 200 and am fond of it

I buy my own printers either from the local recycler or the state's surplus program.  At any given time the universities are essentially giving away quite expensive printers (and the supplies kits) at just a few bucks each, e.g below.  Dinna ask me why, it's just my tax dollars at work.  The nice thing about these workstation printers is that they are actually designed to be fixed when something goes wrong.

https://www.wisconsinsurplus.com/cgi-bin/mmlist.cgi?rlust170/74397

It's a 6-7 year old printer

My Brother printer's older than that and works fine

As mentioned above, your pickup rollers could need cleaning or replacing, but also check your paper setting on the tray itself

If you're using standard sized paper, the tray's paper selector should be set to "LTR" with the width guides pretty tight against the paper.
If it's set to A4 (or other size) or has too much room on the sides of the guide(s), then it can cause pick up issues.

If you're looking to replace the printer, you (IMO) can't go wrong by looking at a Brother

They've never let me down

I decided on the Epson, which will be delivered today.   Thanks to all.   Incidentally, I found a temporary solution.   I had a can of rubber roller rejuvenator that I used  on the pick-up roller and it seems to have solved the problem, at least for now.   It appears to be a petroleum rather than an alcohol-based product and is available from a number of vendors.   I hope this helps someone else as a temporary solution.