Relationship must be on the same number of fields with the same data types. (Error 3368)

The answer to your question is in the error message.

To create and enforce a relationship between two tables, that relationship must be on fields with the same datatype: Number to Number, Short Text to Short Text with the same number of characters in each value, and possibly other datatypes although that would be highly unusual.

So, look at the fields for which you are trying to establish a relationship. Are they both Long Integers? Both Short Text? Something else?

Also, you'll want to be sure that fields which are designated as the Primary Key for a table have the unique property set true. The related field in the second table doesn't need to be unique and generally would not be.

Maybe a screen shot of your tables in design view, along with an explanation of the reason for the relationship, would help.