Thanks for reaching out.
If you buy a domain through Shopify, or transferred your domain to Shopify, then you should have the option to set up email forwarding. If you are not seeing a way to Manage the domain, it may be possible that the verification for the domain has not been completed. I would suggest checking your email spam and junk folders to see if you have received an email with instructions to verify the domain.
Could you let us know if the domain was purchased directly from Shopify, or when it was transferred to your Shopify store? As this will determine the next steps to take for troubleshooting.
Looking forward to your response.