ls_rbls:
"The only difference in screenshots is the 'Modern user interface' which can be enabled and disbaled through Edit-->>Preferences--->>> Signatures."
I can't find anything about "Modern user interface" here. Sure it isn't somewhere else under Preferences?
I'm more than half of that 150 yr all by myself. 😉
I apologize, I forgot to add that the "Modern user interface" was added in Acrobat workspace after your version. So your version still uses the classic interface when signing.
In the screenshots below I disabled the Modern user interface to illustrate that there is no difference between versions of Acrobat... maybe just some minute differences in the steps:
- In my screenshots below I am using the same steps as you but I disabled the the modern user interface so you can appreciate the similarities of the classic user interface just like in the version that you're using

- Below is the end result after I choose the desired graphic to be imported to the digitalID's appearance

And if I access these same settings from EDIT--->>>PREFERENCES--->>>SIGNATURE this is how it looks with the modern user interface disabled in Adobe Acrobat Pro DC (paid subscription):

Note that there is no difference from the screenshots that you provided earlier. That is what I meant by Acrobat version been irrelevant. So it looks like we are missing an step.
But if none of this guidance works , I would say to click on Help and "Repair Installation", then try again.