You created multiple full backups in .tibx format. Then some of the backups were deleted or moved to a different location.
When you try to validate the backup using Validate all versions option, validation fails.
Specifics of product implementation. If some of the backups were moved or deleted outside Acronis True Image interface, then the integrated backup structure will be broken, resulting in the inability to validate the backup, as some parts of the backup archive are missing.
You can validate the latest backup slice using quick validation: go to the backup menu and select Validate the latest backup version in the drop-down:
For best results, manage backup files via Acronis True Image interface: see Acronis True Image: How to delete old backups