Once Operating system image is imported into SCCM, it must be uploaded or deployed to Distribution points. So that it can be used to image computers.
During an Operating System Deployment process, client initiates a deployment connection and tries to access those files on Distribution points using Network Access Account. If an Operating system Image is not found then it will throw an error.
So make sure, you deploy all operating system Images which are going to be used by client machines.
In SCCM console, Under Software Library > Operating Systems
Right click on Required OS image and Click Distribute Content.
Click Next to Continue
On Specify the Content Destination screen, Click Add and Click Distribution Point.
Select the Required Distribution Point and Click OK.
Once the Required DP is selected, click Next to Continue
Click Next to Confirm the Settings
Click Close on Wizard completion.
This completes the Deploying Operating System to Distribution Point so that clients machine can be installed properly.