I'm having similar problems installing R packages for Ubuntu 18.04.
I've tried the different solutions presented in this thread without sucess.

What do you recommend I try next.

I've posted below my output results when running system check.

Thank you for your help

-------------------------------- general ---------------------------------
architecture: 64bit
system: Linux
release: 4.4.0-18362-Microsoft
version: #836-Microsoft Mon May 05 16:04:00 PST 2020
distribution: Ubuntu 18.04 bionic
number of CPUs: 4
apparent login shell: bash
shell RC file: .bashrc (exists)

--------------------- AFNI and related program tests ---------------------
which afni : /home/chammo20/abin/afni
afni version : Precompiled binary linux_ubuntu_16_64: Aug 21 2020
: AFNI_20.2.14 'Aulus Vitellius'
AFNI_version.txt : AFNI_20.2.14, linux_ubuntu_16_64, Aug 21 2020
which python : /usr/bin/python
python version : 2.7.15rc1
which R : /usr/bin/R
R version : R version 3.6.3 (2020-02-29) -- "Holding the Windsock"
which tcsh : /usr/bin/tcsh

instances of various programs found in PATH:
afni : 1 (/home/chammo20/abin/afni)
R : 1 (/usr/bin/R)
python : 1 (/usr/bin/python2.7)
python2 : 1 (/usr/bin/python2.7)
python3 : 1 (/usr/bin/python3.6)


testing ability to start various programs...
afni : success
suma : success
3dSkullStrip : success
uber_subject.py : success
3dAllineate : success
3dRSFC : success
SurfMesh : success
3dClustSim : success
3dMVM : success

checking for R packages...
rPkgsInstall -pkgs ALL -check : FAILURE

oo Warning:
These packages are not installed on the computer: brms!


R RHOME : /usr/lib/R

checking for $HOME files...
.afnirc : found
.sumarc : found
.afni/help/all_progs.COMP : found

------------------------------ python libs -------------------------------
++ module loaded: PyQt4
module file : /usr/lib/python2.7/dist-packages/PyQt4/__init__.pyc

++ module loaded: matplotlib.pyplot
module file : /usr/lib/python2.7/dist-packages/matplotlib/pyplot.pyc

-------------------------------- env vars --------------------------------
PATH = /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/mnt/c/Program Files/WindowsApps/CanonicalGroupLimited.UbuntuonWindows_2004.2020.812.0_x64__79rhkp1fndgsc:/mnt/c/Program Files (x86)/Intel/Intel(R) Management Engine Components/iCLS/:/mnt/c/Program Files/Intel/Intel(R) Management Engine Components/iCLS/:/mnt/c/WINDOWS/system32:/mnt/c/WINDOWS:/mnt/c/WINDOWS/System32/Wbem:/mnt/c/WINDOWS/System32/WindowsPowerShell/v1.0/:/mnt/c/WINDOWS/System32/OpenSSH/:/mnt/c/Program Files (x86)/Intel/Intel(R) Management Engine Components/DAL:/mnt/c/Program Files/Intel/Intel(R) Management Engine Components/DAL:/mnt/c/Program Files (x86)/Intel/Intel(R) Management Engine Components/IPT:/mnt/c/Program Files/Intel/Intel(R) Management Engine Components/IPT:/mnt/c/Program Files/Intel/WiFi/bin/:/mnt/c/Program Files/Common Files/Intel/WirelessCommon/:/mnt/c/Program Files/IBM/SPSS/Statistics/Subscription/JRE/bin:/mnt/c/Program Files/IBM/SPSS/Statistics/25/JRE/bin:/mnt/c/Users/bmc/AppData/Local/Microsoft/WindowsApps:/mnt/c/Users/bmc/AppData/Local/Polycom/RealPresence Desktop/:/snap/bin:/home/chammo20/abin:/home/chammo20/abin

PYTHONPATH =
R_LIBS = /home/chammo20/R

LD_LIBRARY_PATH =
DYLD_LIBRARY_PATH =
DYLD_FALLBACK_LIBRARY_PATH =

------------------------------ data checks -------------------------------
data dir : found AFNI_data6 under $HOME
top history: 20 Feb 2020 [rickr]: updated FT_analysis examples
data dir : found AFNI_demos under $HOME
top history: 22 Oct 2019 [discoraj]: added Cluster Explorer Demo
data dir : found suma_demo under $HOME
top history: ...s_New/data/Build_tmp on Mon Mar 4 11:56:45 EST 2013
data dir : found afni_handouts under $HOME
atlas : found TT_N27+tlrc under /home/chammo20/abin

------------------------------ OS specific -------------------------------
which apt-get : /usr/bin/apt-get
apt-get version : apt 1.6.3 (amd64)

have Ubuntu system: Ubuntu 18.04 bionic
have Ubuntu afni : Precompiled binary linux_ubuntu_16_64: Aug 21 2020

========================= summary, please fix: =========================
* just be aware: login shell 'bash', but our code examples use 'tcsh'
* missing R packages (see rPkgsInstall)video