I assume you refer to the Home ribbon tab. When you right click, are you seeing all of the other AutoCAD Architecture panels other than Build, or are you seeing the out-of-the-box vanilla AutoCAD Home ribbon panels?
AutoCAD Architecture panels: Build, Draw, Modify, View, Layers, Annotation, Transparency (not turned on), Inquiry, Section & Elevation, Details
AutoCAD panels: Draw, Modify, Annotation, Layers, Block, Properties, Groups, Utilities, Clipboard
If you are seeing the vanilla AutoCAD panels, then you likely are running AutoCAD Architecture "as AutoCAD" and the acad.cuix file is your main customization file. The Build panel will not be found there. In the Options dialog, on the Profiles tab, is the AutoCAD profile current? If so, do you have any profiles that start with AutoCAD Architecture? For example, I have four, based on an installation done in the United States, in which UK content was included: AutoCAD Architecture (Global), AutoCAD Architecture (UK), AutoCAD Architecture (US Imperial) and AutoCAD Architecture (US Metric). If you have some like that, try setting one current and see if that fixes your Build panel issue. If your previously current profile had already been highly customized, you can leave that set current, and then go to the Files tab > Customization Files node > Main Customization File node and change the file from acad to aca and see if that restores what you want, without losing other settings.
If you are running an AutoCAD Architecture profile, check the Options dialog, Files tab > Customization Files node > Main Customization File node. Is it set to acad or aca? If acad, try changing it to aca and sett if that restores your Build panel. If it is set to aca, then your aca.cuix file may have been edited to remove Build panel from the Home ribbon tab. (I hope that the Build panel was not deleted.) Using the CUI command, you can check which panels are assigned to the Home panel, and, if the Build panel exists, add it back. If this is your situation, and you want more details on using the CUI command, post back and ask.
David Koch
AutoCAD Architecture and Revit User
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