There are exactly three reasons why you can't describe an object that generate this exact message.
Not Enabled In Org
For standard objects that can be enabled in Setup (e.g. Quote), or by Sales/Support (e.g. WorkOrder), you can get this error. If it's a standard object, see if you can describe it as an admin. If not, it's not enabled, and you need to visit a Setup page somewhere, or contact Sales or Support to get it enabled.
Not In Production
For custom objects that are created in the org, if the Status is set to "In Development," only users with "Modify All Data" and "View Setup & Configuration" permissions can view the object. This is true even if the user has CRUD access; if they are not Administrators, they can't see it. Check this on the Object's detail page in Setup, or check its
deploymentStatus and make sure it's set to
Missing Read Permission
For standard and custom objects in the org, if the user lacks at least Read Permission, they cannot query or describe the object. Check the user's Profile, or add a Permission Set that includes at least View access to the Object.
There are other errors that can occur, but are related to different types of permissions.
Can Not Be Described
sObject type 'X' is not supported.
The object was recognized as a valid object in the org, and the user has permission to it, but it's describe flag is turned off, so you can't actually see the contents of the object.
No such column 'X' on entity 'Y'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names.
The user has access to the object, but the field X either doesn't exist, or the user doesn't have View access to the field.
API Not Enabled For Org
The X API is not enabled for this Organization.
Your org doesn't have the correct permission to use a particular API.
API Not Enabled For User
API is disabled for this User.
The user doesn't have API access (API Enabled is set to false).
There's lot of other possible messages a user can get as well, but the point of this answer is to state that there are only a very limited number of circumstances where this exact error message can occur.