There are a couple problems with your code that I can see. First, the FindFirst method doesn't work on a table-type recordset. It will only work on a dynaset-type or snapshot-type recordset. To find a record in a table-type recordset you need to use the Seek method.
Second of all, if you change the type to dbOpenDynaset, your code will give you an "Invalid argument" error on the FindFirst method. The reason for this is because the argument for the FindFirst method must contain a field name, comparison operator, and a value. You have only given it a value to search without telling it what field to search in. Example:
Let us know if this helps.
Set db = CurrentDb()
Set rs = db.OpenRecordset("tblTrackingSheetFrm", dbOpenDynaset)
StrProjectNo = Me![ProjectNumber]
rs.FindFirst "ProjectNumber = " & StrProjectNo
'If the data type of the field ProjectNumber is Text then use:
'rs.FindFirst "ProjectNumber = '" & StrProjectNo & "'"