Hello Danusia,
Firstly, I apologize for the late reply. You can use Wild Apricot using a subdomain, however your emails will be membership@membership.corbamtb.com - you would need to ask your domain provider to set up an email for this subdomain and then forward it to your main email address.
If your SPF is showing as invalid, you;ll need to ensure it is set up on your subdomain and not on your main domain.
Matthieu (Ex-Apricot)
Please contact support directly for assistance on any of my posts as I will no longer be checking these forums: support@wildapricot.com