Could be two reasons for that. You could be trying to upload the wrong version of the plugin. Trying to upload a 1.5 plugin to a 2.5 site (or vice versa) will give you that message.
Also if your tmp directory is not set correctly in the global configuration, or if your tmp directory is not writeable you will get this message.
Check the path to the tmp folder in global configuration, be sure it's correct and that the tmp folder actually exists. Make sure it's CHMOD 755. If you have Akeeba Admin tools you can fix the permissions using that.
Check it all out and let us know.
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upgrading from Joomla 1.5
1 year 1 week ago #48741