hello checkbox not showing when selecting paid services in php8

I would say you need to update font awesome family or font weight, should be fixed in latest version of plugin, but there could be some incompatibility in theme.

You're right. I have the Gamma theme installed and the flags do not work in it. in the Beta theme, checkboxes are displayed.

@Vlad7
As I told usually you just need to update font-family or font-weight, not quite sure why they made it such difficult to migrate from font awesome 4 to v5... and I see now that v5 to v6 is also complicated, as v6 looks closer to v4.

I solved the problem this way: In Osclass_Pay/css/user.css folder
there are two lines

.osp-options label > div:after {cursor: pointer; text-indent: 0px; font-family: "FontAwesome","Font Awesome 5 Free"; content: "\f00c"; color: transparent; font-size: 13px; width: 18px; height: 18px; line-height: 18px; text-align: center; font-weight: 500; text-shadow: none; position: absolute; top: 0px; left: 0px; border-radius: 2px;}
.osp-options label > div:hover:before { border-color:rgba(0,0,0,0.25);background:rgba(255,255,255,0.15);}
.osp-options label > div:hover:after {cursor: pointer; font-family: "FontAwesome","Font Awesome 5 Free"; content: "\f00c"; color: transparent; font-size: 13px; width: 18px; height: 18px; line-height: 18px; text-align: center; font-weight: 500; text-shadow: none; position: absolute; top: 0px; left: 0px;border-radius: 2px;}

font-weight: 500   Need to change to   font-weight: 900;

Same with Delta theme.

Thanks for sharing feedback