New version upload on TestFlight after prior version is already on App store

I have developed app and so many times I have uploaded on iTunes connect (TestFlight). Like :

Version Build

1.5 —> 1.0 This build is live — on App Store

1.4 —> 1.9
1.4 —> 1.8
1.4 —> 1.7
1.4 —> 1.6

1.3 —> 1.9
1.3 —> 1.8

and so on……

Now I need to update one change and want to give for testing to my other friends, So I am trying to upload new version `1.5 —> 1.1’ on iTunes connect (TestFlight) through Xcode, but it gives error like : Version must be higher than the existing version on iTunes connect.

Before making live, I have uploaded so many build for the same version and same via Xcode , at that time it is done successfully uploaded.

Then why it gives me error now ?
What it means to say ?
How can I upload on TestFlight for testing ?

Please suggested me, what should I do.