Time-limited OpenCore Plus cores problem

Does that error occur immediately after you download using the Quartus programmer? The tools are telling you that you need to keep the programming cable connected as well leave the nag message that pops up on the screen otherwise the hardware will timeout and stop operating.

If the Nios II core is the only thing in your system requiring a license then you could work around this by switching the cpu to the 'e' core. The e core doesn't require a license and as a result will not cause Quartus II to generate the time limited .sof file. The performance of the 'e' core is much lower than the 'f' core due to the lack of caches and pipeline.