First you shouldn't need to enter anything in the TLS Config node if the Brokers certificate is signed by a public CA.
Secondly the TLS Config node supports 2 modes
File path mode, where the path you enter has to point to a file on the machine the Node-RED backend is running. This may not be the same as where the editor is running in the browser
Upload mode, where you upload the certificate file from the machine with the editor.
The error implies you have provided a file path from your editor machine and it can't be found on the machine running the backend.
It's also worth noting that by default NodeJS applications don't need to be passed Public CA files as it has a built in Certificate Store.