Either the extension you use to login does not have a corresponding button in the button configuration matching that extension number, or your user does not have permissions assigned. Most probably is the former.

If you have for example this in your button config file:

[SIP/something]
type=extension
extension=100
context=default

Then you must log with extension 100 in order to perform actions with SIP/something device, so your user line should be something like:

user=100:password:all

Best regards,