The installation procedure of the older FLEUR version is actually pretty different. Basically, you should first adopt the IMakefile, then run imake and if needed change the resulting makefile. Please note that this might require detailed knowledge of how to compile on your system. But let me also point out, that as a new FLEUR user I would strongly recommend against trying to use the older version as little documentation is available, and thus it will be quite difficult to learn how to use this version.

Can you actually indicate why you need to run v0.26?

Daniel