If you want to use 'lengthMenu' together with buttons(copy, export), you have to use this option dom: 'lBfrtip'.
Here https://datatables.net/reference/option/dom you can find meaning of each symbol.
For example, if you will use like this 'Bfrtip', lengthMenu will not appears.
According to datatables.net the proper way to do this is adding the lengthMenu property with an array of values.
If you want to use 'lengthMenu' together with buttons(copy, export), you have to use this option dom: 'lBfrtip'. Here https://datatables.net/reference/option/dom you can find meaning of each symbol. For example, if you will use like this 'Bfrtip', lengthMenu will not appears.