bootstrap-tagsinput.scss

//
// bootstrap-tagsinput v0.8.0 for Bootstrap v4.0.0-alpha.6
// --------------------------------------------------


.bootstrap-tagsinput {
    width: 100%;
    cursor: text;
    background-color: $input-bg;
    border: $input-btn-border-width solid $input-border-color;
    color: $input-color;
    font-size: $font-size-base;
    @include border-radius($border-radius);
    @include box-shadow($input-box-shadow);
    @include transition($input-transition);
    padding: 4px 6px;
    line-height: $input-line-height;

    & input {
        border: none;
        box-shadow: none;
        outline: none;
        background-color: transparent;
        padding: 0 6px;
        margin: 0 2px;
        width: auto;
        max-width: inherit;

        &:focus {
            border: none;
            box-shadow: none;
        }

        &::-moz-placeholder {
            color: $input-color-placeholder;
            opacity: 1;
        }

        &:-ms-input-placeholder {
            color: $input-color-placeholder;
        }

        &::-webkit-input-placeholder {
            color: $input-color-placeholder;
        }
    }

    &.form-control {
        & input::-moz-placeholder {
            color: $input-color-placeholder;
            opacity: 1;
        }

        & input:-ms-input-placeholder {
            color: $input-color-placeholder;
        }

        & input::-webkit-input-placeholder {
            color: $input-color-placeholder;
        }
    }

    & .badge {
        margin-right: 2px;
        font-size: 75%;
        @include border-radius($border-radius);

        & [data-role="remove"] {
            margin-left: 8px;
            cursor: pointer;

            &:after {
                content: "x";
                padding: 0px 5px;
                background-color: rgba(0, 0, 0, 0.1);
                @include border-radius($border-radius);
            }

            &:hover {
                &:after {
                    background-color: rgba(0, 0, 0, 0.62);
                }

                &:active {
                    box-shadow: inset 0 3px 5px rgba(0, 0, 0, 0.125);
                }
            }
        }
    }
}

Update variables :
$input-color-placeholder become $input-placeholder-color
$input-line-height become $input-btn-line-height

Quick fix :

// Fix variables name
$input-color-placeholder: $input-placeholder-color;
$input-line-height: $input-btn-line-height;