Percona restore databases error
The error message is quite explicit:

Error: datadir must be specified.


The documentation is clear on this too:


  --datadir=DIRECTORY The source directory for the backup. This should be the same as the datadir for your MySQL server, so it should be read
  from my.cnf if that exists; otherwise you must specify it on the
  command line.
  
  When combined with the --copy-back or --move-back option, --datadir
  refers to the destination directory.


Note the last sentence, and make sure you're providing the correct path.

The documentation is here.