This is an issue with the resource pack at the minute, I believe it may be fixed by now? However, if it isn't just play on 1.12 for now (1.12.2 is the best version for the server) and try deleting all of your server resource packs and they will automatically re-download when you enter the game - to do that, go into your .minecraft folder, server-resource-packs, delete everything and rejoin.
Hope this helped!