Command rejected An interface whose trunk encapsulation is Auto can not be configured to trunk mode

This error message means that you cannot configure a switch port to be in "trunk" mode if the trunk encapsulation is set to "Auto". This is because the switch cannot determine which trunk encapsulation protocol to use if it is set to "Auto". To resolve this issue, you need to first set the trunk encapsulation protocol to a specific type, such as "802.1Q" or "ISL", and then configure the port to be in "trunk" mode.