Hi. You'll need to do some line-by-line walk throughs of your code to see what is going on. Set a break point at, say, the IF statement in line 13 and then single-step through the code looking at the values of all fields to see what is going wrong.
I'd surmise that if all is working OK until line 21 - the update statement - then the update itself failed. This can happen if you have attempted to insert a record that breaks a relational constraint on the table concerned, or that would create a duplicate key for example.
Regarding the 'item not found in this collection' error, this is telling you that the field name concerned ([Replacement Positions]) is not correct - either the field does not exist in one or both of the two recordsets, or it is spelled differently. Why the error message mentions 'collection' is that when using recordsets the VB compiler is iterating through the fields collection of the recordset to match the names of the fields as you have listed them; if it can't find a match it generates the error you have mentioned.
For a more detailed guide on how to use the VBA editor's debugging facilities please see our articles on
Debugging in VBA.
-Stewart