Open the second tab which is "SOLIDITY COMPILER" and select the solidity version that matches with the solidity version written in file as "pragma solidity …..". The version should be equal to or higher than the file’s version. For example, in my file, "pragma solidity ^0.6.0" is written so the required version is 0.6.0 or higher. As shown, in the compiler the solidity version is 0.6.6, which is ok. After checking the solidity version click the compile button. If you did not change anything in the file, or the solidity version is not wrong, the contract should compile without any errors.