Key 'SYSTEM0000000000' is a Taxon_Designation_Set key and this set will not include your locally created taxon_Designation_Type.

Ideally, you should create a totally new Taxon_Designation_Set which includes the Taxon_designation_Types in SYSTEM0000000000 plus your locally created one. There are various tools available in Reports and Batch Updates which make this a fairly simple process and there is  comprehensive docuementation available. http://jncc.defra.gov.uk/pdf/rec6_Designations-TechnicalTutorial.pdf

Alternatibvely you could just add your Taxon_Designation_Type to SYSTEM000000000 by manually adding a line into table Taxon_Desigation_Set item.

Taxon_Designation_Set_Item_key = LCA0000900000001
Taxon_Designation_set_key = SYSTEM0000000000
Taxon_Designation_type_Key = (The key for your local designation type)
Entered_By = TESTDATA00000001
Entry_Date = now
System_Supplied_Data = false (or 0 if your use Management Studio)
Custodian = (Your Site Id)

You would then need to rebuild the Taxon_designation_Index

The only drawback to this approach is that you might  find your entry gets deleted in a future JNCC dictionary update.