Hello, coincidentally, I am on the same lab, 6.3, and am experiencing the same issue.
I can read the file with vi editor, but cannot write to it.
Also, echo This is a new line >> /tmp/afile is getting a permission denied result.

My commands are:
setfacl -m u:testuser:rw /tmp/afile

getfacl /tmp/afile
file: tmp/afile
owner: janderson
group: janderson
user::rw-
user:testuser:rw-
group::rw-
mask::rw-
other::---

testuser@Ubuntoo2204:~$ cat /tmp/afile
This is a file

testuser@Ubuntoo2204:~$ echo This is a new line >> /tmp/afile
-bash: /tmp/afile: Permission denied

Okay, my thought was that the /tmp directory was not allowing x(executable) for the 'other' permission.
On the /tmp directory, I changed the 'other' or 'world' permission from t(sticky)bit to x via chmod o=rwx /tmp. The original for o was rwt.
I can now write to the afile file in the /tmp directory.

I put the t(sticky)bit back onto the /tmp directory for I don't want to change any global/system settings on the directory.

I tried adding the testuser to the facl on the /tmp directory with rwx, still unable to write to the afile file just using the facl feature by itself.

I moved the afile to my home directory. My home directory is allowing x for 'world'.
I added testuser to the facl with rw- on the afile and am able to read/write to it.
When I remove testuser from the facl, rw permission is denied as expected.

I don't know if using a system directory is the best way to test the facl feature. But, I'm new to all of this, so I may be way off.

@luisviveropena
That worked for me as well!

How are you guys using the second window, after sudo su - fool, to even GET to the first window/user's directory? I can't cd in and ls tells me the directory doesn't exist (which is right, because I didn't make a home/fool dir).

Realized that I needed to cd into the directory BEFORE sudo su , then ls lists "afile".