Apple Hardware Test (AHT) will not start...

No Apple software is responsible for putting the .diagnostics directory back on the boot drive. And in later version of MacOS, it is exceptionally difficult to do so manually.

This was such a problem that one user decided to collect all the Apple Hardware Test files he could find and make them available on a GitHub site as well as directions for making a bootable USB-stick diagnostics drive:

https://github.com/upekkha/AppleHardwareTest

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the correct PRAM Reset procedure for your model Mac:

Hold four keys at startup:

Command-Option-P-R

and hold until you have heard the chimes three times, then release.

the SMC Reset keyboard sequence is only useful for battery-capable Macs. If your Mac Pro 2010 has been off power for more than a few minutes, it will automatically do a System Reset, including System Management Controller (SMC) Reset. You can 'help it along' by pressing and holding the power button for quarter minute while the AC power cord is disconnected.

if you can't invoke some 'snag keys' sequences such as Option-Boot, remember that these require the use of a wired keyboard directly on the chassis (from or back) not a SSB port on a display or other Hub.

The Apple Hardware Test was embedded on the original HDD and on Disk #2. If the original HDD is still working, connect it to the Mac and see if you can invoke the Apple Hardware Test.

In leu of the missing Apple Hardware test, run EtreCheck Pro.

see > https://etrecheck.com/en/index.html

Then post back here with the report if you require help deciphering that report.

see > https://discussions.apple.com/docs/DOC-250000211

What year model and type of Mac are were working on?

If the Mac came with an Install Disc set, then the Apple Hardware Test can be found on Disc #2.

If the Mac is newer and did not come with an Install Disc set, then the Apple Hardware test that was on the original Hard Drive is gone.

After the Mac is up and running from the SSD, try going to Apple > System Preferences > Startup Disk and make sure that the Macintosh HD on the SSD is selected as the Startup Disk.

If that does not help, then please spend more time describing in detail, what SSD you used in that model Mac and how it is formatted.

You require NOTHING more that the free version of Etrecheck.

You can post the report here for Readers to help you analyze. The shareable report is already 'pre-laundered' of any personally-identifiable information, so safe to post. Instructions can be found within the program.

The Developer is a senior contributor here. Etrecheck was born from frustration at 'playing 20 questions' trying to solve problems here. But it takes Time and Energy to Develop, and Server power to Host the binaries, so the modest add-ons and Pro version are a way to start to recover some of the cost of development/hosting.

Their add-on services are NOT REQUIRED to use the base version. If you like them, they are one-time charges, and not very expensive. If Etrecheck helped you solve a problem, it is worth the modest cost of an upgrade, or a modest 'tip-jar' contribution.

As per Apple recommendation: Use Apple Diagnostics to test your Mac – Apple Support (AU)

Use Apple Diagnostics to test your Mac:

Apple Diagnostics, formerly known as Apple Hardware Test, can check your Mac for hardware issues.

If you think your Mac might have a hardware issue, you can use Apple Diagnostics to help determine which hardware component might be at fault. Apple Diagnostics also suggests solutions and helps you contact Apple Support for assistance.

Prepare your Mac

1. Shut down your Mac.
2. Disconnect all external devices except keyboard, mouse, display, Ethernet connection (if applicable) and connection to mains power.
3. Make sure that your Mac is on a hard, flat and stable surface with good ventilation.

Start Apple Diagnostics

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

1. Turn on your Mac and continue to press and hold the power button as your Mac starts up.
2. Release when you see the startup options window, which includes a gear icon labelled Options.
3. Press Command (⌘)-D on your keyboard.

Intel processor

1. Turn on your Mac, then immediately press and hold the D key on your keyboard as your Mac starts up. 
2. Release when you see a progress bar or you're asked to choose a language.

That Mac Pro 5,1 can't boot faster from a cold start with 128 GB of RAM. The Power-on-Self Test including RAM test takes a long time, (way more than ten seconds) and will delay the initial Chime until it passes.

MalwareBytes is the best-respected malware DETECTOR on a Mac. but you generally don't get any benefit from running is scanning your files non-stop. I suggest you turn that feature off.

Flip4Mac is obsolete, because Windows Media Viewer lost out to other formats and is mostly gone. I think built-in support for H.264 or H.265 took its place.

you may to have a remnants of a Cisco VPN and NordVPN installed. If this is required by your Institution, that is great, and they can provide support. It is generally a mistake to install your own VPN except in very special circumstances. it does NOT improve protection.