Sometimes you might not able to install Spectra plugin via WordPress dashboard. In such case, try installing it via FTP.

Below are the detailed steps –

Step 1 – Download Spectra plugin zip from WordPress repository
Step 2 – Unzip the plugin zip file
Step 3 – Open your WordPress site via FTP. We would recommend you to use FileZilla
Step 4 – From FTP, visit the root of the WordPress install
Step 5 – Now visit wp-content folder > plugins
Step 6 – Upload Spectra unzipped file from step 2
Step 7 – Now the plugin is installed on your site. Go to WordPress Dashboard and activate it from Plugins page.