Some time ago I had a similar issue, for some reason the installer would not automatically unmount earlier created still active/ mounted partitions (older test installations), and would later report an error or simply fail to proceed, back then it somehow worked to go a step back, reinit hardware and disk detection after which as far as I can remember it would come up with a prompt to unload active partitions and you could continue partitioning.
Some time ago I had a similar issue, for some reason the installer would not automatically unmount earlier created still active/ mounted partitions (older test installations), and would later report an error or simply fail to proceed, back then it somehow worked to go a step back, reinit hardware and disk detection after which as far as I can remember it would come up with a prompt to unload active partitions and you could continue partitioning.