I had a server that me and my friends have made ages ago for 1.12.2,  I decided to download the world file to play on by myself but I can't interact with any kind of block inside the TARDIS, and the TARDIS exterior doesn't appear to exist anywhere in the world

the tardis probably doesn't belong to you which would be why you cannot interact with it, there is a command you can use to change the owner, alternatively you can just change the config file for it and assign it to yourself. as of the exterior use the summon command to summon the interior.