When I try to update my Iphone 7plus it continues to say can't update

Assuming you mean updating the operating system, there are several reasons why you may be having trouble with this.

You do not have enough storage on your phone for the update? You can check this on your iPhone using this article. https://support.apple.com/en-us/HT201656

Please bear in mind that while the update is only about 2-3GB in size, it requires about 6-8GB in space for full access.

Another reason you may have difficulty is that you may not be connected to wifi. Also, you will want to make sure you have sufficient battery life to complete the update.

Finally, you may find this article helpful. https://support.apple.com/en-us/HT204770