MySQL need to make column NOT UNIQUE. Error Can't DROP 'ColumnName'; check that column/key exists

MySQL need to make column NOT UNIQUE. Error Can't DROP 'ColumnName'; check
that column/key exists

ColumnName is Unique (UNIQUE KEY ColumnName).
I just want to make column not unique (must be very simple, but can not
understand how).
If in phpMyAdmin check at column name and at bottom click on Unique icon,
get #1062 - Duplicate entry '' for key 'RegistrationNumber'. OK, see it is
because, clicking on icon it ADD UNIQUE.
There is Unique icon in Structure within row. But the icon is not click-able.
As in phpMyAdmin did not found how to do it, trying with query.
Based on advices tried ALTER TABLE TableName DROP INDEX ColumnName.
Get 1091 Can't DROP 'ColumnName'; check that column/key exists
Here http://stackoverflow.com/a/4414694/2465936 found This error means
that you are trying to delete a key which is being used by another table.
Possibly the ColumnName is used by another table.
Please advice what need to do to make column not unique.