If you originally had only the SSD installed, you should be able to enter your system firmware (UEFI/BIOS) and alter the boot order to ensure that the SSD receives higher boot priority. This would be due to it having been the only drive present at the time of installation and thus having the System Partition with the Boot Configuration Database (BCD) installed upon it. Some system firmware will automatically prioritize hard drives over SSD, regardless of what port they are connected to.
If another drive was installed alongside the SSD and has now been removed, this drive could have contained the system partition with the BCD and would result in the error you have described upon its removal. To repair this error without re-inserting the old drive, you would need to use the BootRec tool from a Windows Recovery Environment. You can access one from your installation media by selecting Repair your computer after confirming your language settings. This would be best done with only the SSD connected. Access the Command Prompt under Troubleshooting and Advanced, then enter the below and press enter to repair the BCD:
More information at Microsoft Support is available here