Hello Yassine,
The problem you specified seems similar to the issue discussed in the following MATLAB Answers thread:
https://in.mathworks.com/matlabcentral/answers/409520-having-trouble-with-matlab-r2018a-installation
It seems like you might have downloaded the R2018a Update 3 installer, which contains only updated files. First, you need to download and install the R2018a installer, then update to the latest version.
Hope this helps.