This error happens when there is old gallery code from ignite gallery version 4 being called with the ignite gallery version 5 code.
The most common cause of this is template overrides in the templates html/com_igallery/category folder. (this is in your site’s template folder)
If you have templates overrides from version 4 of Ignite Gallery, you will need to remove them. Version 5 of ignite gallery is a major new release that supports Joomla 5. The Version 4 code can not work with the version 5 code.
If you need to customise the layout, please email support first, Matt might be able to send some custom css that works better than template overrides.
If the above info does not resolve your issue, please go to the Joomla global config, turn on debug mode, and then see which file the error comes from, then email support.