Troubleshooting

Failed to Download Resource

Just uploaded changes to the website.

Check you have uploaded all the individual files.
Forgot to upload the ribbon bundle file.

Starting Runtime

Trying to run the add-in from local host is unable to start the runtime.

Failed to Install Custom Functions

Clearing out the Task Pane addins using Document Inspector and resaving the file
Opening the file again displays the following

Error Installing Functions

Function names must be unique.

Error loading add-ins

The ribbon commands appear but the custom functions are not available.

One or More Add-ins failed to load

Changed the GUID in the manifest file before clearing out the WEF folder first.

Add-in Error - New Office Add-in

Steps to reproduce:
Delete the contents of the "WEF" folder.
Close Excel and ReOpen Excel.
Open the "_User_Defined_Functions.xlsm" file.
When it says this add-in comes from a shared folder, it is referring to the location of the manifest file.
link - https://stackoverflow.com/questions/37900413/microsoft-excel-addin-how-to-know-if-its-already-installed-office-js

Add-in Error - No Longer Available

After removing any old custom functions that were referenced in the active workbook this message still appears.
This also appears if you have a workbook that contains functions but the manifest file cannot be loaded.

Pressing F2 and pressing Enter does return the correct value for existing formulas but the value displayed is #VALUE!.

Resolved by running the Document Inspector and clearing the Task Pane add-ins.
Check the add-in is being displayed on the Shared Folder of the Add-ins dialog box.

Add-in Error - Ribbon Callback Error

After making some changes to the manifest file in the "shared-catalog" and saving them while Excel was open.
Pressing on any of the ribbon buttons displays this message.

My changes have invalidated the manifest file.
Check the add-in is being displayed on the Shared Folder of the Add-ins dialog box.
Check the following file: C:\Personal\Addins\shared-catalog\ OfficeAddinsLog.txt

Add-in Loads but Ribbon is not displayed

The manifest file is valid but the ribbon does appear.
This will happen if you ribbon contains a Menu control that has No Items

<Control xsi:type="Menu" id="Information_Men_ID"> 
   <Label resid="Information_Men_Label"/> 
   <Supertip/> 
   <Icon/> 
   <Items> 
   </Items> 
</Control>