I'm not entirely sure why, but I have my suspicions. Would like to point out that Discord has been expiring these types of links for that last few months. Ask any questions below for further assistance or to request another how to article. Now just ensure your set to never expire and check step 2 maybe you want to specify which room to start them in before clicking create link. When you click 2 you will be brought to the next screenshot and finish there. The expiration setting is on here 2 times as seen donot panic. Click again on create instant invite as shown. Now your screen should look like this next screenshot. Then at the top left menu button as pictured here, click on it. First click open discord and join the server you want to get an invite to.