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Did you install AutoCAD Architecture, and not "vanilla" AutoCAD? If yes, are you running AutoCAD Architecture as AutoCAD Architecture (using one of the out-of-the-box profiles) or are you running it "as AutoCAD"? (Did you start the program from a desktop shortcut or did you just double-click on a DWG file in Windows Explorer?)
What Custom User Interface [CUI] file is being used for the main CUI file on the computer that does not have the Build panel? is it ACAD.cuix or ACA.cuix? (You may know this, but in case you do not, you can check the name of the main CUI file in the Options dialog, on the Files tab, Customization Files node, Main Customization File sub-node.)
I assume when you right clicked on the ribbon, you chose the Show Panels flyout when checking for the potential presense of the Build panel, and found it was not there.
Does the balance of your Home tab consist of the AutoCAD Architecture version? [Build], Draw, Modify, Layers, Annotation, Inquiry, Section & Elevation, Details, View and Transparency, rather than the vanilla AutoCAD Draw, Modify, Layers, Annotation, Block, Properties, Groups, Utilities and Clipboard. Are the rest of your ribbon tabs the ACA ones or vanilla AutoCAD ones? Does your Manage ribbon have a Style & Display panel?
Open the Customize User Interface dialog (CUI command), and, on the Customize tab, in the upper left pane, expand the Workspaces node under the ACA node. Select the workspace that has "(current)" after its name, and in the upper right pane, expand the Ribbon Tabs node. Is the first tab called Home - ACA (rather than Home - 2D, Home - 3D or Home - 3D Basics, or some other name)? If not, then you may be using a Home tab that was imported from the ACAD.cuix.
If you have not made many customizations, you might want to open the Customize User Interface dialog (CUI command) and, on the Customize tab, in the upper left pane, right click on the ACA node and choose Reset ACA.CUIX from the context menu. Or, if you have customizations you would prefer not to lose, and you have a Build - ACA panel defined (upper left pane, ACA > Ribbon > Panels > Build ACA), you could try to add it back to Home - ACA ribbon tab (upper left pane, ACA > Ribbon > Tabs > Home - ACA).
If Home - ACA is not in the current workspace, then you will need to add it. Select the current workspace in the upper left pane (under ACA > Workspaces). In the upper right pane (Workspace Contents), select the Customize Workspace button. Back in the upper left pane, under ACA > Ribbon, expand the Tabs node and scroll down to the Home - ACA item. Click on box in front of that item to add a check mark to the box and add the tab to the current workspace. In the upper right pane, expand the Ribbon Tabs node to verify that Home - ACA is present and to move it to the top of the list, if necessary. Select the Home - ACA item in the upper right pane and review its properties in the lower right pane. Verify that the Show property (Appearance category) is set to Yes. If you are done making workspace changes, select the Done button in the upper right pane to end workspace editing mode. Select OK to ratify the changes and return to the drawing.
If you cannot find the items above in the CUI editor, then your ACA.cuix file may be beyond repair, and Doug's suggestion of a repair install or full reinstallation may be in order.
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