Does someone know why this error happens?

From my understanding, these warnings displayed are not the Error causing the synthesis to fail. These warnings shown are simply saying that no circuit is driving the various wires (such as wbs_ack_o)... When compiling the User Project that's expected, as the Wrapper drives those signals from the caravel. Dig down into the directory shown in the log/synthesis folder. There should exist an errors.log file (not warnings.log) that will give details on exactly which error is causing the compiler to fail.