Additional, debugging information is needed to help understanding the problem. Please follow these steps:

1. temporarily configure your system as follows:

   $g_display_errors = array(
   E_WARNING => 'halt',
   E_NOTICE => 'halt',
   E_USER_ERROR => 'halt',
   E_USER_WARNING => 'halt',
   E_USER_NOTICE => 'halt'
   );
   $g_show_detailed_errors = ON;

WARNING - SECURITY RISK: the 'show_detailed_errors' config can cause MantisBT to display sensitive information about your system. We recommend to restrict its activation to a Test environment, only for as long as necessary. If possible, do not turn it ON globally, instead limit it for specific user(s) using the Manage Configuration page.

2. Reproduce the error

If the output does not help you resolving the issue and you need further assistance,

3. save the output of the error screen, preferably as an HTML page (not a screenshot !)
4. be careful to remove/mask any confidential information if needed
5. upload the results here

Also provide details about your setup (PHP version, database type and version), and check for any additional error information in the PHP log file.

This is the error you are getting in the Summary section

error en pagina resumen.png (120,747 bytes)   

error en pagina resumen.png (120,747 bytes)   

The error was solved by installing the sudo yum install php-json dependency on the virtual machine, and enabling it in the php.ini file. Thanks to the visualization of the error, it was solved, thank you very much.

@userAnonimo the admin guide is telling to run admin/check after installation or upgrade, see https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.install.postcommon
There you would have seen that the PHP extension is needed.