When you create a Synapse workspace its important to either add yourself and other AD accounts using Azure CLI task. See my github page for this. https://github.com/clintgrove/azure-synapse-bicep-v2
You can also see what I have done in this screenshot
Basically, you are creating the Synapse workspace with a service principal, but you are no where to be featured, so you have to add yourself. In the arguments on line 48 you will see a GUID, that is my object ID in Entra ID