You can assign permissions to SharePoint group using SharePoint REST API like:

function setPermissionForGroup(){
    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/roleassignments/addroleassignment(principalid=<GroupId>, roledefid=1073741827)",
        type: "POST",
        contentType: "application/json;odata=verbose",
        headers: { "Accept": "application/json; odata=verbose","X-RequestDigest": $("#__REQUESTDIGEST").val(),"X-HTTP-Method": "POST" },
        success: function (data) {
            alert('Contribute permission set on group');
        },
        error: function (data) {
            alert("Error: " + JSON.stringify(data));
        }
    });
});

Replace <GroupId> with the ID of SharePoint group.

roledefid for Permission Levels are as follows:

• 1073741829: Full Control

• 1073741827: Contribute

• 1073741826: Read

You can assign permissions to SharePoint group using SharePoint REST API like:

function setPermissionForGroup(){
    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/roleassignments/addroleassignment(principalid=<GroupId>, roledefid=1073741827)",
        type: "POST",
        contentType: "application/json;odata=verbose",
        headers: { "Accept": "application/json; odata=verbose","X-RequestDigest": $("#__REQUESTDIGEST").val(),"X-HTTP-Method": "POST" },
        success: function (data) {
            alert('Contribute permission set on group');
        },
        error: function (data) {
            alert("Error: " + JSON.stringify(data));
        }
    });
});

Replace <GroupId> with the ID of SharePoint group. Open the SharePoint group & you will find the group ID in browser URL:

roledefid for Permission Levels are as follows:

• Full Control: 1073741829
• Design: 1073741828
• Edit: 1073741830
• Contribute: 1073741827
• Read: 1073741826
• Limited Access: 1073741825
• View Only: 1073741924

You need to convert above REST call in Power Automate Send an HTTP request to SharePoint action.