Answer:
Allow VLAN 20 on the trunk link.
Step-by-step explanation:
Enable DTP on both ends of the trunk by allowing VLAN20 on the trunk link.
From curriculum topic: 3.2.4
To Enable DTP on both switches is simply allowing the negotiation of trunking. The "Negotiation of Trunking" line in the graphic is what shows that DTP is already enabled. The graphic also shows how the native VLAN is 1, and the default VLAN for any Cisco switch is 1. The graphic shows the PCs are to be in VLAN 20.
This will remedy the problem.