Http 400 is just a general error thats generally due to invalid syntax.
maybe if you share your code, the error logs, and what you're trying to achieve through these calls, some of the other members of the community more well versed in scripting/api requests can help you out! make sure to add some screenshots
You may either ask your developers what this error might mean or use some recording proxy (Fiddler for example) to record the same request sent from some other tool (SoapUI, for example) when it succeeds and from TestComplete when it fails. Analyse the differences and try to eliminate them. (For example, some required header might be missed in the request sent from TC.)
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