Third Party SSL - The certificate is not trusted in all web browsers

Hi Everyone,
I have not come across this problem before and i believe i have setup the SSL according to the instructions i have found online or within this forum. Unfortunately i am still getting the following error

The certificate is not trusted in all web browsers. You may need to install an Intermediate/chain certificate to link it to a trusted root certificate. Learn more about this error. You can fix this by following Sectigo's Certificate Installation Instructions for your server platform (use these instructions for InstantSSL). Pay attention to the parts about Intermediate certificates.

I am also using litespeed server.

Thanks

Bossmanuk