Hi,
Are you attempting to assign your Custom Role at one of the scopes you allowed in assignable scopes? For example, if when you created the custom role you only listed the subscription under assignable scopes tab, then the role will not show in the list if you attempt to assign it to a resource group.
If you would like to add assignable scopes you may navigate to your subscription -- Access control (IAM) blade, Roles tab, then click the context menu (three dots) on the right of your custom role and choose Edit.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP