Go to Virus & threat protection > Protection history . If it is there, restore it.
The Steam-api64r.dll file is essential for the proper functioning of the Steam client and games on 64-bit Windows systems. Without this file, you may experience issues such as: Steam-api64r.dll File Download
Placing unverified system-level files into Windows folders can corrupt your operating system registry. Go to Virus & threat protection > Protection history