The error message tells you that you do not have a program named ledger
installed in your exec-path
.
If you do have a program named ledger
installed somewhere then add its path (location) to the value of exec-path
.
See the Elisp manual, node Subprocess Creation
for information about exec-path
.
See also environment variables PATH
and EMACSPATH
in the Emacs manual, node General Variables
.