Hello,
The Pagseguro payment plugin is not an official HikaShop plugin but a open source plugin created by "Jobadoo Webdesign" and available in GitHub :
github.com/HikaShop/hikashoppayment-pagseguro
The warning messages are just warnings and not fatal errors. They just indicates that your Joomla logs folder is not writable ; so the plugin cannot create his log file into.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.