Here are links to DDS plugins. I had problems finding a 64 bit plugin that worked with PSP 2021. It is very basic but works for court and uniform files and also has a 32 bit version. I uploaded the plugin that worked for me to NLSC. Intel TextureWorks or nVidia may work for you.
DDS Plugin 64/32 bit
https://developer.nvidia.com/nvidia-tex ... s-exporter
https://software.intel.com/content/www/ ... lugin.html
https://forums.getpaint.net/topic/11173 ... 0-08-2020/
https://gimp-dds-plugin.software.informer.com/