There are several possible reasons:
Check the "behavior" settings for the field on the layout. You can set a field up so that it can't be entered in browse mode.
Double click the field in browse mode and check the table occurence name for the field. (It's the drop down just above the list of fields.)
Compare this name to the name shown in Layout Setup... (Show records from) If they are different, there could be an issue there.
I have 2 tables: one is invoice and the other is payment.
Invoice::ID_Invoice_pk is related to Payment::ID_Invoice_fk.
In a layout that is showing records to Invoice table, i have a field from the Payment table called "Payment".
This payment field is an edit box and the box to edit in browse mode is marked. Even though the invoice layout that i'm in is related to the payment table that the payment field is in...the layout will not allow me to enter in text to the payment field that is on my invoice layout.
Pardon me for misunderstanding your original post...
Is this an empty field and you are attempting to enter new data? If so, check the relationship details in Manage | Database | Relationships and make sure "Allow creation of records via this relationship" is enabled for your Payment table.