We are allowed to contest an election legally; for example, filing the lawsuits and entering objections during the counting of electors.
But, when we submit false electors, try to pressure the VP to violate his constitutional duties, urge state officials to "find" votes, and conspire to overturn the election, it becomes illegal. That is especially true when Trump knew he lost and all the claims of fraud had no evidence. He was told that by family, staff, and administration. He lost most of the court cases because they presented no evidence.
If Trump's supporters did not believe his lies January 6 would never have happened.