Error: Activation Rejected. (Code: 400)

Symptoms

Attempt to activate a license fails with error Error: Activation Rejected. (Code: 400):

licensing.log:
[I 56/00000001/T0F18/P0D44/S0001] 02-04-20 16:31:48 - POST URL: https://ras.parallels.com/licensing_api/v1/activate, Headers: Authorization: Token bd88e87b72454b95af6731086ed31573, Query: {"host":{"hw_id":"FC9C923C756FAA5FEBDB9251C24F02F9","os_name":"Windows Server 2016 Standard Edition (x64) ","name":"PR-RAS-01","new_hw_id":"9EEC7069BF18EF535D42E8E4921403AF","product_version":"17.1.21681"},"farm":{"name":"TESTNAME","guid":"91743C7B250F4141BB353498FF7B5930","publishing_agents":[{"name":"TESTNAME","ip_addresses":["TESTNAME"]}]},"lic_key":"JQ726-xxx-yyy-zzz-aaa","comment":"","force":false,"offline_act":true}
[I 56/00000001/T0F18/P0D44/S0001] 02-04-20 16:31:49 - POST try 1 response: Return code: 400. Body: {"errors": {"lic_key": "pattern mismatch", "error_list": [{"value": "JQ726-xxx-yyy-zzz-aaa", "path": ["lic_key"]}]}, "msg": "invalid parameters", "details": ""}.

Cause

Invalid license key - e.g., a symbol or two are missing.

Resolution

Repeat the activation with a valid license key.