The file \AppData\Roaming\npm\yarn.ps1 is not digitally signed. code example
Example: The file \AppData\Roaming\npm\yarn.ps1 is not digitally signed.
Remove yarn.ps1 from the directory C:\Users\%username%\AppData\Roaming\npm\ then try clearing the npm cache at C:\Users\%username%\AppData\Roaming\npm-cache\