Run the command line version of R from the same terminal where you deactivated conda, and install from there. So, all from the same terminal, one by one:
conda deactivateRinstall.packages('igraph')
Do not use RStudio for the installation. You can use RStudio later once installation succeeded.
Let me know if this worked. If not, can you post the error message along with the preceding compilation command, just like you did before?
For the future, ideally, set up conda in a way that it is not active by default. I also use conda, but only activate it when I use it. See the last paragraph here.