This message is displayed because the server settings for upload max size or maximum memory are not well configured.
Don't worry, this is a common issue, very simple to fix.
Install the plugin manually by adding the contents of the plugin zip file to the
wp-content/plugins folder on your server via FTP.
If you are not used to set your server php / memory settings, contact your hosting company to do it for you. This is their job, it takes 1 minute to do.
Show them a screenshot of your error, or email them the error message. Ask them to assist by increasing the following values, until you are able to install your theme or upload your image.
memory_limit upload_max_size post_max_size upload_max_filesize max_execution_time max_input_time
If you know how to change the php and memory settings of your server, try to update the following values:
memory_limit = 256M upload_max_size = 64M post_max_size = 64M upload_max_filesize = 64M max_execution_time = 300 max_input_time = 1000