Api call branch_protections returns "The target couldn't be found."

I’m trying to use the Rest API to set branch protections.

Tests are all done via Swagger. Other Rest API calls work without problems.

But if i call [Patch] branch_protections, it returns “The target couldn’t be found.”

When i set an invalid owner, i get a proper message so i think the owner is not the problem.

I do have an organisation named testorg which has a repository testrepo
The repository has a branch named testbranch

The following is the curl command shown by swagger.

curl -X 'PATCH' \
  'https://myservername.com/api/v1/repos/teamcity/testorg%2Ftestrepo/branch_protections/testbranch' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
  "approvals_whitelist_teams": [
    "string"
  ],
  "approvals_whitelist_username": [
    "string"
  ],
  "block_on_official_review_requests": true,
  "block_on_outdated_branch": true,
  "block_on_rejected_reviews": true,
  "dismiss_stale_approvals": true,
  "enable_approvals_whitelist": true,
  "enable_merge_whitelist": true,
  "enable_push": true,
  "enable_push_whitelist": true,
  "enable_status_check": true,
  "merge_whitelist_teams": [
    "string"
  ],
  "merge_whitelist_usernames": [
    "string"
  ],
  "protected_file_patterns": "string",
  "push_whitelist_deploy_keys": true,
  "push_whitelist_teams": [
    "string"
  ],
  "push_whitelist_usernames": [
    "string"
  ],
  "require_signed_commits": false,
  "required_approvals": 0,
  "status_check_contexts": [
    "string"
  ],
  "unprotected_file_patterns": "string"
}'

This is the response i’m getting
image

Any idea what is wrong, or what is meant by target?