The HDRP / URP shaders in Gaia require a certain editor preference to be set in order to be able to compile correctly. The shader graph variant count needs to be set to 256 or higher in order for these shaders to work. You should be prompted to increase the variant count during the Gaia Maintenance / Installation process if it is lower than 256 with this prompt:
This setting however requires a restart of the unity editor before it applies. This means when you did not increase the setting during the initial install, the HDRP / URP shaders will not be compiled properly and they will not be found during the shader install process for those pipelines.
Gaia will warn you if it detects this problem in the Gaia Manager when you are about to perform the shader install, but the setting has not been set yet:
Fix: This problem can be fixed by performing the following steps one after another:
1. Open The Preferences via Edit > Preferences and increase the Shader variant limit to 256 or higher:
2. Restart the unity editor
3. Reimport the folders
Assets\Procedural Worlds\Flora\Content Resources\Shaders
by right clicking on them and selecting "Reimport".
4. Perform the shader installation now, it should continue through the end now and the button for the shader install should disappear at the end when the install was successful.