Peering connection

Based on your description, there are a few potential causes for the peering connection failure between your Cloud WAN Core network in Account 1 and the Transit Gateway in Account 2:

1. Region mismatch: The transit gateway must be in the same AWS Region as the core network. If they're in different regions, the peering connection will fail.

2. ASN conflict: The Autonomous System Number (ASN) of the transit gateway and the core network must be different. If they have the same ASN, this will prevent the peering connection from being established.

3. Static route configuration: Cloud WAN only supports dynamically propagated routes for transit gateway connections. If you're trying to add static routes, this will cause an error.

4. Permissions issues: Ensure that both accounts have the necessary permissions to create and manage the peering connection, even after sharing the Cloud WAN Core network.

5. Route table configuration: After establishing the connection, you need to properly configure route tables in both networks to direct traffic between them.

To troubleshoot further, verify these settings and ensure that all prerequisites are met. If the issue persists, checking the BGP logs for any errors or warning messages might provide additional insights into what's preventing the peering connection from being established.
Sources
Peerings in AWS Cloud WAN - AWS Network Manager
prerequisites to ensure vpc peering | AWS re:Post

The issue may be due to overlapping Autonomous System Numbers (ASNs) between your Transit Gateway (TR GW) and Cloud WAN ASN ranges. To resolve this:

Check your Cloud WAN ASN range:

Go to Core Network > Policy Version Look under the Network Configuration tab If the range is from 64512 to 65534, your Transit Gateway needs an ASN outside this range Change your Transit Gateway ASN:

Select your Transit Gateway From the Actions menu, choose "Modify transit gateway" Change the ASN to a number to for example something between 4200000000-4294967294.

This should resolve the conflict and fix the issue.