Hi @Jill Collins5, thanks for trying our new Community!
For replacement question, as you mentioned, since your 3 seats of Internet Security are all taken, you cannot install on the 4th PC unless you disable one of registration of your current PC. If want to transfer the license to another PC, here's an online doc to help you manage the transfer: "Transferring a license from one device or computer to another"
Please let us know if the solution works for you. If you're still encountering problems, please provide a description or picture of the message, we'll be glad to assist you. Thanks.