Used InfinityFree's SSL certificate tool to implement certificate's details, but still having issues

[ChrisPAR]

Unfortunately, as mentioned by @KangJL GTmetrix will not work with SSL on InfinityFree, as they require the intermediate chain certificates to be installed.

GTmetrix error on a brandnew Joomla 4 test site

This issue is actually related to this: For some strange reason, GTMetrix requires an intermediate/chain certificate (which is not supported on free hosting) to be installed in order to perform the test, as explained on their site: General GTmetrix Test Errors | GTmetrix