It is best if you use the same name for your sitemap as for your file.
if default.sitemap, then sitemap default label="Our Home" {
or our_home.sitemap with sitemap our_home label="Our Home" {
you have another issue with chromecast. post some logs from openhab.log using code fences
edit:
This Frame (on line 23) is missing the closing bracket on line 34
Frame {
Text label="Light" icon="light" {
check out VS Code openHAB Extension!