Venmo app displays on my iPad sideways

Not all Apps are coded to operate in both Portrait and Landscape screen orientation. If the App does not support one or other screen orientation, there is nothing that you can do to work around this limitation.

You might consider contacting the App Developer - and requesting that the App be updated:

How to contact an app developer – Apple Support

If the App does not support one or other screen orientation, there is nothing that you can do to work around this limitation.

Whilst it costs you nothing to try, unless the App supports both screen orientations, reloading the App will not resolve the issue.