Hello Harry,
We need more information about the issue and it should be investigated.
1. Please enable debug mode / set Display Errors to Maximum and repeat the issue.
2. See error.log on server side to examine what's inside.

Also you can provide temporary credentials to access your site to us for further investigation of the problem.

Best regards,
Eugene

I have checked installing this version of payment plugin on Joomla 5.3.1 and Phoca Cart 5.1.0 and all works fine.
Thereby, this error is related to your server environment.
There might be different reasons. Anyway, a deep investigation needed.
1. Make sure you have the correct permissions and owner on folders/files (755 for folders and 644 for files and the owner should be correct)
2. See sever side error.log. It will give more detailed info about the issue.
3. Conflict with 3rd party extension inside the Administrator Panel. Are there any 3rd party extensions that "protect" your website? Something like guard/shield software. Maybe it blocks some operations.
4. Do you have core .htaccess file or maybe it was modified by some extension like Admin Tools? (maybe some rules prevent installation process)
5. You can try to install any other package (extension) to make sure that this issue is global.

If no success will be reached, you can send us temporary access (Administrator Panel and FTP) to your site for investigation.

Harry,
There might be absolutely any reason why this issue occurs since this is web. And we should determine the reason. I checked out on my side and there is no such issue. It is hard to help by seeing the screenshot. Something blocks the installation.
I have already described various ways to check out. But no information followed from you. Did you check out other advices? Please follow my recommendations above and provide more information. If you do not want to provide access on your site, I see 2 alternative ways to go:
- Contact hosting provider and ask them to help.
- Create a subdomain on the same server/hosting, install clean Joomla, Phoca Cart and our plugin to make sure all works fine or the issue is the same.
- Examine error in browser console.

500 POST /administrator/index.php?option=com_installer&task=install.ajax_upload HTTP/1.0

The error means that an internal server error (HTTP 500) occurred when trying to install an extension via Joomla's installer. This is not a Joomla-specific error but rather a server-side or PHP-level issue related to the upload process.

Possible Causes and Solutions:

1. Exceeded file size limits (upload_max_filesize / post_max_size)
If you're uploading a large ZIP extension manually, the server may cut off the request due to size restrictions, resulting in a 500 error.

Solution:
Check and increase PHP settings:

upload_max_filesize = 32M
post_max_size = 32M
max_execution_time = 300

Restart the server after editing php.ini.

2. Hosting limitations

Some hosting providers restrict POST requests or limit what scripts can be run in the admin area.
Solution: Contact your hosting provider and ask if there are any restrictions on POST requests to administrator/index.php.

3. Incorrect file/folder permissions

Joomla may not be able to write temporary or installation files.

Solution:

— Check permissions on tmp, logs, and the extension folders (administrator/components, components, plugins, modules, etc.).
— Set directories to 755 and files to 644.
— Make sure the PHP process has ownership of the files (e.g., www-data, apache).

4. Check error logs

To diagnose further, check the server’s error logs:
— Apache: /var/log/apache2/error.log
— Joomla: logs/error.php (if logging is enabled)

5. Conflict with 3rd party extensions.

Make sure there are no conflicts with any 3rd party extension. Examining the browser console can help to get more details.

Alternative Installation Method

If the web-based installer fails:

— Unzip the extension locally on your computer.
— Upload the folder to Joomla’s /tmp/ directory via FTP.

In Joomla:

— Go to Extensions → Manage → Install from Folder
— Enter the path: /tmp/your_extension_folder
— Click Install.

If you can share your error logs or enable Joomla debug mode, I can give you more specific advice.

Harry,
We got the access. Thank you. Now We are examining the issue. I will let you know details asap.

Best regards,
Eugene

Dear Harry,
Thank you for reporting thios issue to us and for providing the credentials. We have examined and confirm: this issue is related to our plugin and the new version of Phoca Cart. The issue occurs under new installation of Phoca Cart 5.1.0 and our plugins.
The fixed version will be released today. I will inform you additionally.

Best regards,
Eugene

Quote:
An error has occurred.
0 Cannot assign null to property PlgPcpStripecoInstallerScript::$fromVersion of type string

Dear Harry,
A new version of the plugin has been released. Changelog
Please update it through Joomla or download it from our site.
Thank you very much for reporting us this issue.
Please let us know about your progress with installing the plugin.

Best regards,
Eugene

Hi Harry,
You're most welcome.
We would be grateful if you will leave a short review about our plugin/service at the JED or on TrustPilot.
Thank you so much and have a nice weeked!

Kind regards,
Eugene