Just download the latest OpenXcom, open the folder, take the openxcom.exe file out and use it to replace your copy of openxcom.exe.
He said he's using Linux, not Windows.
And just replacing exe (or its linux alternative) is exactly what can lead to such errors... always do a full proper installation with all files, not just exe!
Also just make sure you're using a nightly build within the last 2 years or so. The builds really are pretty stable as they come out. But make sure you aren't using 1.0. That version is WAY too old.
He also said he's using nightly.
And the interface change was done approx. 1 month ago, so he must be using a recent one.
Or perhaps better yet, do a complete re-install by deleting the folder contents except for the game files (original x-com files) and then extract everything from the latest nightly version of openxcom.
Yes, I can recommend this too.
Delete everything and start fresh.
Could you please explain how I can fix this? I am still new to OpenXcom and Linux.
Thank you
What I was trying to say is that your installation is broken.
Without seeing your files, the only fix I can give you is "delete everything and install fresh".