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Try using the debugger. It will help you pinpoint the problem.
The code field is primary key field and in other table, Fields are connected with this field using Table Relation.
The problem might be that when you try to rename field "Code" shown in screenshot, it also goes to update other field value whose are connected with "Code" field in Table relation Property. You should check for all fields and their data type which are connected with "Code" field and their data type.
I think data type of any one field does not match with the data type of "Code" field .
If suggestion has solved your problem then please do not forget to mark it as solved.