You could remove the justify-content center and add margin to center horizontally.
Because the width is only 60%, the text can be centered inside of the div, but the div containing the text (your tab card) is only 60% of its container (tabcontent). If you don't specify otherwise, the tab card will be put at the beginning (left most) of tabcontent and only take up that 60% width. By saying margin 0 auto you're saying you want 0 margin in the top and bottom, and on the left and right equal margin to center the div within it's container.
See this css:
.tabcontent {
padding: 6px 12px;
border-top: none;
}
.tab-card {
text-align: center;
margin: 0 auto;
margin-top: 80px;
width: 60%;
}
If you would like to use justify-content, you would need to justify the contents of the container to center. Additionally that container would need to have the display of flex. See below.
.tabcontent {
display: flex;
padding: 6px 12px;
border-top: none;
justify-content: center;
}
.tab-card{
text-align: center;
margin-top: 80px;
width: 60%;
}