We are working on Magento ver 2.2.3 and inserted a logo into transactional emails, but it never work.
We have the same error over and over, we tried to look on the vendor theme path, (there is not header.html, could be that the error?
I guess the easiest way, without playing with any code is to manually add the logo image:
1) create the directory
2) copy the logo to the directory manually using FTP.
3) Then via the correct procedure Content > Design > Configuration > Edit > Transnational emails >
Once finished, upload the same logo you manually added to the newly created directory.