System: Ubuntu 16.04 LTS, yet you can try these steps in accordance to your respective systems.
If there is an SDK file present, it should be most likely found at /home/USERNAME/Android/sdk
USERNAME is to be replaced by your username
If there is none, check the specified sdk path for the project in android studio.
File > Project Structure > sdk path
The sdk directory should be present in the specified path. In case, it is not there, open the file:
PROJECT_DIRECTORY/android/local.properties
PROJECT_DIRECTORY needs to be replaced by your project name.
If the file is not there, create it. Then add the following line depending on where you find the sdk directory.
If sdk is there at /home/USERNAME/Android/:
add the line: sdk.dir = /home/tanya/Android/sdk
If sdk is not there at /home/USERNAME/Android/:
add the line: sdk.dir = /home/tanya/Android/
If the path specified for sdk directory in 'Project Structure' is entirely different and the sdk directory is present at the specified location,
add the line: sdk.dir = SPECIFIED_SDK_PATH
Add the specified sdk path in place of SPECIFIED_SDK_PATH