Why cant i install adobe reader dc if i already have the full version of acrobat installed?

You should read this topic:

https://community.adobe.com/t5/acrobat-discussions/reader-and-acrobat-on-same-pc/td-p/12083571

@DK263 

In addition to was Jeff has suggested-

After the Acrobat Reader migration to 64-bit  If you have installed Acrobat Reader 64-bit and you purchase an Acrobat subscription, the Acrobat Reader 64-bit application will upgrade to become a fully functional Acrobat 64-bit application. 

Hence,  you can only keep one at a time

https://helpx.adobe.com/acrobat/kb/about-acrobat-reader-dc-migration-to-64-bit.html 

Thanks,

Akanchha 

DK263, thanks for your question. Reader and Acrobat are primarily the same software, with different features made available depending on the licensing utilized. If you want to use a modern version of Adobe Reader, please uninstall any older versions of Acrobat from the computer. Please see https://helpx.adobe.com/download-install/kb/uninstall-reinstall-acrobat.html for information on how you can remove an older full version of Acrobat, to install a newer, more secure version of Reader to view your PDFs.