Hi Mariusz,
You can use 'Evaluation Mode' to generate time_limited *.sof programming files, please refer the ' 2.1.1. Intel FPGA IP Evaluation Mode' from the below FFT IP core user guide,
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/ug_fft.pdf
Please keep backup of the project, delete 'db' & 'Incremental_db' directories from the project directory then remove IP from project & add again the IP.
By default enabled Intel FPGA IP Evaluation Mode feature check below link.
https://www.intel.com/content/www/us/en/programmable/support/support-resources/download/megacore-ip.html
And for buying IP please refer the below link.
https://www.intel.com/content/www/us/en/programmable/buy/intellectual-property.html
or
Please refer to Intel® FPGA Licensing Support Center to get started, find information on license types, getting a license file, setting up a license file, and resolving license-related issues.
https://www.intel.com/content/www/us/en/programmable/support/support-resources/support-centers/licensing.html
Otherwise for license file request and activation, log in your “My Intel” account to find Intel FPGA Self Service License Center and submit Contact Customer Service form.
Regards,
Vicky