Npm string empty when Etherpad is loaded as Service

Question

today Iam trying to get an Etherpad Instance working as a Service.

The Installation and configuration of the Etherpad is fine. Everything works well if I start the Etherpad myself.

But I want to get this running as a Service on autostart.

The Etherpad wont start due to the following Error:

 Abort trap
 ERROR: Cannot extract npm major version from version string ""
 [: : bad number
 [: : bad number
 ERROR: Your npm version "." is too old. npm 5..x or higher is required.
  1: 0x16adf70 node::Abort(void) [/usr/local/bin/node]
  2: 0x16811da node::PlatformInit(void) [/usr/local/bin/node]
  3: 0x16801a0 node::InitializeOncePerProcess(int, char**) [/usr/local/bin/node]
  4: 0x1680779 node::Start(int, char**) [/usr/local/bin/node]

Any tips?

Edit:

Iam using the following Versions:

• Etherpad: git master
• npm 6.14.8
• nodejs 15.6.0