The goal of the vaccine is to train the immune system to recognize the real virus when it shows up. If you get it wrong, the vaccine may not simulate the virus

The goal of the vaccine is to train the immune system to recognize the real virus when it shows up. If you get it wrong, the vaccine may not simulate the virus very accurately which can lead to a number of issues. You may not mount an immune response when the virus shows up, or worse the vaccine may have improperly trained your immune system and you create antibodies that are ineffective against the virus but wreak havoc on your body. This has happened before with vaccine candidates like RSV or Dengvaxia. It can be deadly. That's not even getting into the issues that may arise from skipping all the GMP/GLP guidelines. They can be tedious but exist for a reason.