I think that Suspend is not available in your Tenant.
How to check whether Suspend is available on your Tenant:
Log in to the Sandbox UI.
Go to Subscription page.
Click "more" in the upper right and check whether there is a link "Suspend".
If you can not see this link on your Tenant:
Suspend (and Resume) is one of the functions called Limited Availability (LA). https://knowledgecenter.zuora.com/DC_Developers/G_SOAP_API/E1_SOAP_API_Object_Reference/Amendment > - Suspend a subscription (This feature is in Limited Availability.) > - Resume a subscription (This feature is in Limited Availability.)
LA is deactivated by default. In order to activate LA, you need to ask Global Suppot.
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