There could be a number of reasons you are unable to send. Some things to check:
-Check that you have the latest version of O3
-Do you have enough of the selected asset to send?
-Are you connected to the internet?
-Are you using a router/company internet that blocks plain HTTP requests (try turning off wifi)?
If all these are fine, it may be a server issue. Try checking out O3.community, to see if others are experiencing similar issues and if someone from the O3 team has posted. If not, feel free to create a new post and we will update you as soon as possible.