Error – Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode

switch(config-if)#switchport mode trunk
Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode.

This is a result of a missing encapsulation command type (That is 802.1Q on that port)
– Enter the interface configuration mode and enter the following command below

switch(config-if)#switchport trunk encapsulation dot1q

Before proceeding with the rest configuration
– The port can now be set as a trunk port without any issue.

switch(config-if)switchport mode trunk