When creating a load balancer, the request will fail if a flavor is specified. Checking the code against the api documentation for Octavia and Neutron, I see that flavor is being used in gophercloud, where the API requires an attribute named flavor_id. I suspect this wasn't discovered as this is a fairly obscure feature.

Debug output from Terraform:

Error: Error creating LoadBalancer: Bad request with: [POST https://openstack-test.bbtnet.com:9696/v2.0/lbaas/loadbalancers], error message: {"NeutronError": {"message": "Unrecognized attribute(s) 'flavor'", "type": "HTTPBadRequest", "detail": ""}}

Octavia Source: https://github.com/openstack/octavia/blob/master/octavia/api/v2/types/load_balancer.py#L55
Octavia API Ref: https://docs.openstack.org/api-ref/load-balancer/v2/?expanded=create-a-load-balancer-detail#create-a-load-balancer

Neutron LBaaSv2 Source: https://review.opendev.org/gitweb?p=openstack/neutron-lbaas.git;a=blob;f=neutron_lbaas/services/loadbalancer/data_models.py;h=d44cc8dd5d543af9ffacb7bc1954f332083fdc67;hb=refs/heads/stable/rocky#l724
Neutron LBaaSv2 API Ref: https://docs.openstack.org/api-ref/network/v2/#load-balancer-as-a-service-2-0-deprecated (docs page currently broken)

I will submit a PR.