Please review the documentation on using "FindChild".
https://support.smartbear.com/testcomplete/docs/reference/test-objects/members/common-for-all/findchild-method.html
The number of parameters used is more than just a single field. For FindChild, you pass in two arrays, one of property names, one of property values. You also can indicate the depth of the search as an optional parameter as well as a refresh option.
Recommendation:
Map your objects under name mapping. Then, instead of "FindChild", you can "telescope" your object tree to something more meaningful like:
Aliases.browser.pageLogin.textUserName
And then reference these items directly. As it is, most likely the UserName field is not a direct child of the "Page" object but is more likely buried a bit deeper in the tree of objects and containers. Again, a good recommendation for getting started with TestComplete and your application under test is to record your actions and then, after you have completed the recording, make modifications, etc., based upon how TestComplete discovered your objects, components, etc.