Hi, 

I have a similar error while trying to install conn on a supercomputing system. 

Matlab is installed here: /usr/local/matlab/r2021a (this matlab folder also has conn in /toolbox/conn)

Conn is installed here: /usr/local/spm12/matlab2021a.r7771/toolbox/conn

I have added the conn path in matlab as follows: 

path(path, '/usr/local/spm12/matlab2021a.r7771/toolbox/conn/')


However, when I try to start CONN, the startup page gets stuck with these error messages (I have tried adding just the spm12 path and also deleting any .m files in my pwd, but those did not help): 

>> conn
ERROR DESCRIPTION:

Undefined function 'spm_fileparts' for input arguments of type 'char'.
Error in conn_existfile (line 31)
[filepath,filename,fileext,filenum]=spm_fileparts(deblank(filename));
Error in conn (line 660)
elseif conn_existfile(filename), dodebug=true; error('INSTALLATION PROBLEM. Please re-install SPM and CONN and try again');
Error in conn (line 135)
conn initfromgui;
CONN20.b
SPM12
Matlab v.2021a
project: CONNundetermined
spm @ /usr/local/spm12/matlab2021a.r7771
conn @ /usr/local/spm12/matlab2021a.r7771/toolbox/conn


ERROR DESCRIPTION:
Unrecognized field name "folders".
Error in conn (line 810)
if ~isfield(CONN_x.folders,'rois'), CONN_x.folders.rois=fullfile(fileparts(which(mfilename)),'rois'); end
Error in conn (line 136)
conn importrois;
CONN20.b
SPM12
Matlab v.2021a
project: CONNundetermined
spm @ /usr/local/spm12/matlab2021a.r7771
conn @ /usr/local/spm12/matlab2021a.r7771/toolbox/conn


ERROR DESCRIPTION:
Undefined function 'spm_fileparts' for input arguments of type 'char'.
Error in conn_existfile (line 31)
[filepath,filename,fileext,filenum]=spm_fileparts(deblank(filename));
Error in conn (line 1033)
if conn_existfile(filename),
Error in conn (line 622)
conn gui_recent_init;
CONN20.b
SPM12
Matlab v.2021a
project: CONNundetermined
spm @ /usr/local/spm12/matlab2021a.r7771
conn @ /usr/local/spm12/matlab2021a.r7771/toolbox/conn

ERROR DESCRIPTION:
Unrecognized field name "filename".
Error in conn (line 1565)
CONN_x.isready(1)=~isempty(CONN_x.filename);
Error in conn (line 640)
conn gui_setup;
CONN20.b
SPM12
Matlab v.2021a
project: CONNundetermined
spm @ /usr/local/spm12/matlab2021a.r7771
conn @ /usr/local/spm12/matlab2021a.r7771/toolbox/conn


ERROR DESCRIPTION:
Undefined function 'spm_unlink' for input arguments of type 'char'.
Error in conn_register (line 15)
spm_unlink(file_registered);
Error in conn (line 642)
conn_register;
CONN20.b
SPM12
Matlab v.2021a
project: CONNundetermined
spm @ /usr/local/spm12/matlab2021a.r7771
conn @ /usr/local/spm12/matlab2021a.r7771/toolbox/conn

Thanks very much. 
EB