Is that notice shown on the browser screen, before (or in the middle of) the SuiteCRM screen, or did you get it in a text log in your server?
XAMPP tends to be very peculiar about PHP errors. The first thing you need to do is to turn off error_reporting in XAMPP, often this is enough to get things going, because this is just a notice, not really an error.
Anyway, check your logs for other errors, the real problem keeping you from logging in could be something else.
Yes it is written before, just below the URL adress.
I have already changed displays_errors and displays_startup :
; display_errors
; Default Value: Off
; Development Value: Off
; Production Value: Off
; display_startup_errors
; Default Value: Off
; Development Value: Off
; Production Value: Off
this is the copy of my logs errors
[Wed Sep 27 00:39:54.492112 2017] [ssl:warn] [pid 5704:tid 556] AH01909: www.example.com:443:0 server certificate does NOT include an ID which matches the server name
[Wed Sep 27 00:39:54.631267 2017] [core:warn] [pid 5704:tid 556] AH00098: pid file C:/xampp/apache/logs/httpd.pid overwritten – Unclean shutdown of previous Apache run?
[Wed Sep 27 00:39:54.778583 2017] [ssl:warn] [pid 5704:tid 556] AH01909: www.example.com:443:0 server certificate does NOT include an ID which matches the server name
[Wed Sep 27 00:39:55.387515 2017] [mpm_winnt:notice] [pid 5704:tid 556] AH00455: Apache/2.4.27 (Win32) OpenSSL/1.0.2l PHP/7.1.9 configured – resuming normal operations
[Wed Sep 27 00:39:55.387515 2017] [mpm_winnt:notice] [pid 5704:tid 556] AH00456: Apache Lounge VC14 Server built: Jul 9 2017 11:48:22
[Wed Sep 27 00:39:55.387515 2017] [core:notice] [pid 5704:tid 556] AH00094: Command line: ‘c:\xampp\apache\bin\httpd.exe -d C:/xampp/apache’
[Wed Sep 27 00:39:55.426048 2017] [mpm_winnt:notice] [pid 5704:tid 556] AH00418: Parent: Created child process 8924
[Wed Sep 27 00:39:56.963899 2017] [ssl:warn] [pid 8924:tid 584] AH01909: www.example.com:443:0 server certificate does NOT include an ID which matches the server name
[Wed Sep 27 00:39:57.172648 2017] [ssl:warn] [pid 8924:tid 584] AH01909: www.example.com:443:0 server certificate does NOT include an ID which matches the server name
[Wed Sep 27 00:39:57.222129 2017] [mpm_winnt:notice] [pid 8924:tid 584] AH00354: Child: Starting 150 worker threads.
[Wed Sep 27 00:40:10.551535 2017] [php7:notice] [pid 8924:tid 1908] [client ::1:50358] PHP Notice: Undefined index: LOGIN_LOGO_ERROR in C:\xampp\htdocs\suitecrm\modules\Users\Login.php on line 120
[Wed Sep 27 00:40:37.377727 2017] [php7:notice] [pid 8924:tid 1868] [client ::1:50368] PHP Notice: Undefined index: LOGIN_LOGO_ERROR in C:\xampp\htdocs\suitecrm\modules\Users\Login.php on line 120, referer: http://localhost/suitecrm/index.php?action=Login&module=Users
[Wed Sep 27 00:40:40.276180 2017] [php7:notice] [pid 8924:tid 1876] [client ::1:50371] PHP Notice: Undefined index: LOGIN_LOGO_ERROR in C:\xampp\htdocs\suitecrm\modules\Users\Login.php on line 120, referer: http://localhost/suitecrm/index.php?module=Users&action=Login
[Wed Sep 27 00:40:56.110434 2017] [php7:notice] [pid 8924:tid 1876] [client ::1:50374] PHP Notice: Undefined index: LOGIN_LOGO_ERROR in C:\xampp\htdocs\suitecrm\modules\Users\Login.php on line 120
Could you, please, know and tell me more about it ?
Are you sure you turned off the errors? Lines starting with semicolon ( ; ) are mere comments, they have no effect. You should have a line below those lines you pasted, with the actual setting.
The messages you show all seem to be notices and warning, none of them are ERROR or FATAL.
Is that notice about the logo still coming up on screen, in the browser?
If your error persists after clearing that, you might have to look into permissions. Try checking the other log, the web server log (called php_errors.log or errors.log, probably).