In Odoo, it happens that a module tries to use functionality of another module, in that case you need to keep that another module in the dependency list of module that is trying to use the functionality. So Lets say there is a module named 'module_A' and other one is 'module_B', where 'module_B' is trying to use some functionality of 'module_A', so we can say that 'module_B' depends on 'module_A'. Here while installing 'module_B' system tries to find 'module_A' and if it doesn't find that it throws the error what you are facing.

Now, as per your statement it says that module 'hr_webcam' depends on 'mymodule' and system is not able to find that module 'mymodule'. Try to check your addons path and make sure you have available that module with you.

Hope this helps !!.