The DTP-5-DOMAINMISMATCH: Unable to perform trunk negotiation on port [chars] because of VTP domain mismatch error message indicates that the two ports in the trunk negotiation belong to different VTP domains. Trunking can be configured only when the ports belong to the same VTP domain. [chars] is the port number.
When trunking mode of a switchport is set to TRUNK and the VTP domain name of the switch is different from that of the neighboring switch, messages indicating VTP domain mismatch and the inability to perform trunk negotiation is printed eventhough the trunk link is established.
This is because DTP packets are exchanged unless they are explicitly blocked. The DTP packets exchanged finds that the domain name mismatches and hence prints the message that trunking is not possible.
To fix this issue, reconfigure the switch to ensure that the two ports that are involved in trunk negotiation belong to the same VTP domain.
Or, configure switchport nonegotiate command under the interface configuration mode in the switch running Cisco IOS to stop the exchange of DTP packets. If the switch is running CatOS, then issue the set trunk nonegotiate command.