Well first, relax! Just say the word verbally, but if baby expresses a continued interest in the object, then take the time to look up the sign and add it to your signing vocabulary. Some words don’t have signs period, and are instead fingerspelled using the ASL alphabet. There’s no reason you can’t teach your baby to fingerspell if you really want to, however this is an advanced skill which won’t really be done properly until your baby is well into their toddler years. Another alternative is to create a homesign, or made up sign. Just realize that you and your baby will be the only ones who know what the sign means so be sure to translate for your caregiver and friends.
Since it’s not necessary to learn all the ASL words anyway, words can be put into broader categories for the purpose of signing. For example, things like “armadillos” can be signed as ANIMALS and if you don’t know the sign for “kiwi” or “avocado”, or another fruit, then just call it a FRUIT. Doing this won’t bother your baby at all and in the long run will help your baby with language. You baby will learn that fruits can be eaten, that most are sweet and that they are often very colourful, and so forth. In other words, they will learn the category for the type of food.
Above: Mom and baby working on the alphabet using ASL signs. The signs are a great visual cue to the letters of the alphabet. When necessary Mom steps in and helps.