Make sure you have a good connection to the Internet. Have you tried using a computer connected to an Ethernet cable?
Read this document if you can’t download or update apps on your iPhone or iPad --> If you can’t download or update apps on your iPhone or iPad - Apple Support
If the first 5 don't help, use the last one, contact Apple.
Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support For some countries look under “Other” listing at the end of each region. Not every country has telephone support.