I got Yunohost all installed sucessfully. I then installed MAtrix Synapse and everything went fine but when I click on the app itself I get this error.
Just to be sure, are you aware that Synapse is a matrix server, with no real user interface ?
You can use any existing Matrix client to access it (for example you can install Element on another domain, or use a client on your smartphone or computer).
On my side, when I try to access Synpse directly, I am redirected to YunoHost user panel.