This is almost always due to the total size of the selected expansion packs exceeding the PSR-S970's 512 MB internal memory. Deselect one or more packs from your installation set in the YEM until the total size is below 512 MB.
The safest and highest-quality source is Yamaha's official store. They offer regional expansion packs specifically optimized for the PSR-S970 architecture. yamaha psr-s970 voices download
Some popular websites for Yamaha PSR-S970 voices download: This is almost always due to the total
Search for "Yamaha PSR-S970 Users" or "Yamaha Expansion Packs" to find community-driven Google Drive links and file shares. yamaha psr-s970 voices download