Directories listed in /etc/synthetic.conf not accessible via the link
First make sure that you have created the file with the correct file permissions:

  Owner: root 
  Group: wheel

  Owner: read+write
  Group: read
  Everyone: read


Make sure that the two columns in your file are seperated by tab characters, and not just spaces.

Additionally the names of your existing folders probably need to be written without the leading slash. At least this is how the man page specifies it, and it is the only way I have tried it - it might work with a leading slash though.

I.e. you need to write Users/steve/data instead of /Users/steve/data.

Next ensure that you have rebooted your system after modifying or creating synthetic.conf. The links are only available after a reboot.