1. Run Safe mode, as per > Use safe mode to isolate issues with your Mac - Apple Support
2. Repair the startup disk, as per > Repair a disk using Disk Utility on Mac - Apple Support
3. Multiple boot partitions, externally connected drives with clones or installed macOS's can also confuse the startup process...
If you still have the problem after completing the above support articles? Try disconnecting all external drives and other devices connected to your Mac. Then go to: Apple > System Preferences > Startup Disks and re-select the desired startup disk that you want the Mac to startup from.