installed on mac os x 10.13.4 got some errors

But the tool still worked...

$ npm i -g @proofofexistence/cli
/usr/local/bin/proofx -> /usr/local/lib/node_modules/@proofofexistence/cli/index.js

dtrace-provider@0.8.7 install /usr/local/lib/node_modules/@proofofexistence/cli/node_modules/dtrace-provider
node-gyp rebuild || node suppress-error.js

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

ACTION binding_gyp_ndtp_target_build_ndtp .
TOUCH Release/obj.target/ndtp.stamp

• @proofofexistence/cli@1.0.3
  added 31 packages in 16.877s

Found the fix

nodejs/node-gyp#569

sudo xcode-select --switch /Library/Developer/CommandLineTools # Enable command line tools

Not sure should do anything about, but noting...

[sesn]

For this issue, Try sudo xcode-select -r. This command worked for me.