Custom apps can only be installed on a single store, can you confirm the app hasn't already been installed elsewhere?
Also if you setup any install requirements for the app (ie. requires merchant located in a certain country, or using a certain gateway), make sure the shop you're attempting to use fulfills those requirements.
If that doesn't explain it, please provide the X-Request-ID from the response headers of the error page in your screenshot, and we can investigate further.
Hey @_JB ,
Thanks for replying (but 16 days after!),
It happened again in our development store. But the installation went successful when I tried again after a few hours. The same thing happened to our client too.
Couldn't reproduce the issue.