This is most often due to a "back up" of model solving for our server. This can occur when there are many solve requests being sent through. Most often canceling and restarting the solve 2-3 minutes after attempting the first one resolves the issue.
One alternative is to try out the Analytic Solver Platform, which also allows models to be solved directly on a users machine. If you have not already, you may start a 15-day free trial by going to www.solver.com and registering for a new account. For technical issues with the Analytic Solver Platform installation or licensing, please start a live chat from www.solver.com or email our support team at firstname.lastname@example.org.
A second alternative to using the Solver App is to use the Standard Solver that is built into Excel, this "solves" directly on your machine so you should not experience issues like this one when using it.
To enable the Standard Solver, please follow the steps available here: