You need to give more information:
WHERE is the file on sys-net? You say you can find it using Files - in
which directory is it to be found?
If you run a command and it gives no reply then the command has finished
without any output. In the case of 'find' it means that no file was
found - this suggests that EITHER the ubuntu image doesnt end in *iso
OR you ran the command somewhere where the file wasnt to be found.
qvm-start is a command to be run in dom0 - you say that you got an
error message - what EXACTLY did you type?
The qvm-run error shows that you have not yet found the location and
filename of the ubuntu image that you downloaded.
You say you downloaded a template and installed it - I assume that you
copied it to dom0 and installed it there?
When you say you "can't find it anywhere" what do you mean?
If you run in dom0 'qvm-ls' you should see an entry for xenial marked
Tpl - If you try to create a new VM then you should see xenial in the
list of availbale templates to use.
If neither is the case, please run this command in dom0:
'sudo dnf list installed|grep template' and confirm that the xenial
template is included in the output.