1. Private URL : Check the URL you entered is private or public  (private : URL can access only in your local environment, can not access in outside of your environment) and public URL (can access the URL from any where). In this situation you can not run execution in Testsigma Lab, you need to choose Hybrid model. How to create Hybrid model execution refer - https://support.testsigma.com/solution/articles/32000021341-how-to-create-a-hybrid-execution-for-running-tests-on-local-machine-

2. Http/Https is missing : Check the URL prefix should either https:// or https://   

3. URL Timeout : If the given application loading more then 30 seconds you will get timeout exception. You can increase page load timeout in execution create page.