This error occur when you are trying to concatenate two fields values into a single formula field or just populating a value from a field in Formula Field which is not returning text.

To avoid this error, use TEXT() method.

Example:


TEXT(CreatedDate) & TEXT(Age__c)


here CreatedDate is a Date Field and Age__c is Number Field.

TEXT() method should be used even in case of Picklist Field.

Cheers!!!