I have made .desktop files for Ubuntu 16.04 to access my Google Chrome profiles, but once I upgraded to Ubuntu 17.10 (it worked up to Ubuntu 17.04) they got removed from the dock and I can't pin them back.
Right click on it and select 'add to favourites'.
If right clicking on application icon in dock doesn't show "Add to favourites" option, search for the application in Activities screen, and then drag it across to the dock (suggested by Legolas).
Note: This is a reference driver for supported DisplayLink USB graphics device that is incorporated in your dock, adapter or monitor. However, please note that your original equipment manufacturer (OEM) provides drivers for your specific monitor, dock or adapter on their website, etc. DisplayLink recommends that you check with your OEM about recommended driver updates for your monitor, dock or adapter. An OEM may not provide technical support for issues that arise from the use of this driver.
Now, the Semi-Automatic Classification Plugin is installed and a dock anda toolbar should be added to QGIS.Also, a SCP menu is available in the Menu Bar of QGIS.It is possible to move the toolbar and the dock according to your needs,as in the following image.