Opening an encrypted document

I saved a document after encrypting it. Now when I open it, I get the above message. The product is Adobe Acrobat Pro 2020 (latest version) on a Windows 10 (latest version). 

I couldn't find the steps to install the Digital ID...but then why do I need to install it - I thought I already had it when I encrypted the document on the same device. The document has not left the device.

Would appreciate any help in resolving this issue.

TIA.

MMJ