Hi everyone,
This message simply means that an add-in (in this case one of the Revit extensions) is not present on the computer on which the file is being opened. There is no bug and no corruption of the file - the message is there to inform you that an add-in made automatic changes to your file previously and that data maintained by the add-in may become out of date if you change the file without the add-in installed.
For example, the reinforcement add-in automatically updates the rebar inside of a column when the column size changes. If you change the size of a column without the add-in installed, then the rebar will not be updated and will end up in the "wrong" size/location. The add-in won't know to automatically update the rebar in this column even if the add-in is used on it later unless the column is modified again.
It you are NOT planning to modify the file, it is completely safe to ignore the warning and open the file.
If you are planning to make changes, you have two options:
1. Install the add-in so that it can continue to update necessary elements.
2. If you are sure that your changes won't affect the add-in or you know that the elements maintained by the add-in will be updated/recreated later anyway, you can continue to work with the file.
Bottom line, the file is completely safe to use, modify etc, just be careful to make sure that elements are properly updated (possibly manually) if/when the file is returned back to an environment where the add-in is present.
Thanks,
Sasha Varsanofieva
Revit Platform Technologies Product Manager
Sasha Crotty
Director, AEC Design Data