truncating transaction log for simple recovery databases
Just because database is in simple recovery does means transaction log will not grow. Actually there is not much difference in terms of logging in full and simple recovery.

In simple recovery model transaction log is truncated when checkpoint is fired and log file grows 70% of its size subject to condition no long running transaction is holding the logs in your case it is the index rebuild which is holding things from being truncated leading to log file increase.

What you can do is use smart index rebuild technique dont rely on SSMS. There are quite few available Ola Hallengren Index rebuild, Minion Index rebuild and My Custom Script.

It may also be possible that the drive on which log file resides needs more space because your indexes are big and needs space and you have to add more space.

Please also read Why Does the Transaction Log Keep Growing or Run Out of Space?