The indexed property of a field is changed from Yes (Duplicates OK) to Yes (No Duplicates) when duplicate data exists in the table.
An expression is not specified in the Expression property of a calculated field.
If the maximum number of locks per file was exceeded, you can increase the number by editing a registry entry. However, this is not a recommended option.
If you use Registry Editor incorrectly, you could cause serious problems that require you to reinstall the operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
Make a backup of the registry. Find the MaxLocksPerFile registry value by using the Windows Registry Editor, and then increase the value. The MaxLocksPerFile value is saved as part of the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\14.0\
Access Connectivity Engine\Engines\ACE
If the Indexed property of a field and duplicate data is located in the table, reset the Indexed property to the previous setting, or remove duplicate records from the table.