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"Input pin x1 of&(9,21) connected to more than one output pin that are not tri state stable"
I get an error like this for each of the pins on the left-most gates in this full adder:

It worked fine independently with button inputs and LED outputs. This error only occurs when I use my full adder as part of a larger adder, such as this one:

It probably telling the truth, you have connected two outputs to one input. If you share here your circuit it will be easier to help you.

Here's the zip of the project. It has a bunch of other simple components, but only Adder and Main were involved in the issue here.

Hi,
What you have is a connection between your inputs A and B on the leftmost adder.
In the future you can search for such incidences by selecting all connected wires Hold Shift key and click a wire and using menu Edit – select all different components.
Hope this help,
Eugene