Solidres v2.13.0 is under heavy development with a lot of changes, we’ve reached a state where it is not possible to support both Joomla 3 & 4 in a single code base. We decided to drop Joomla 3 support in Solidres v2.13.0 and focus only on Joomla 4+ from now on.
From our own experience, upgrading from Joomla 3 to Joomla 4 is not always a straight forward process, so please plan for the upgrading carefully, always make a full backup before upgrading. Upgrading should not be done in your live/production site, you should perform the upgrading in a staging/cloning site first.
We also have a good news for our template users: Greenery v2.0.0 with Joomla 4 support is 90% done, we are finalizing the last details as well as testing it against Solidres v2.13.0. Please check our blog regularly for more news.