Hi,

To upload a custom image you must provide a direct link to its file. Your link does not meet this requirement. To check it, run following command (I am assuming that you are running some Linux OS):

curl -O https://www.dropbox.com/s/4u50g4p4mrkip8b/Bliss-OS.vmdk

Downloading process will be finished quickly. Then, check file’s size:

ls -l Bliss-OS.vmdk

You will get output like that:

Output
-rw-rw-r--. 1 user user 138 Mar 18 22:03 Bliss-OS.vmdk

Your image file’s size is 138 bytes. Look what it contains:

cat Bliss-OS.vmdk

Here is the result:

Output
301 Moved Permanently
The resource has been moved to /s/raw/4u50g4p4mrkip8b/Bliss-OS.vmdk;
you should be redirected automatically.

As you can see, it is not a direct link to an OS custom image file.
For comparison (and for a test) you can try to download VirtualBox OS image using following direct link:

curl -O https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.vmdk

Again, the file will be downloaded to your current directory. If you check its size, you will see it is 514MB, what seems to be OK for Linux distro image file. You can use this link to upload an image to your DO account. It will work.