you can not place blocks in the nether roof in bedrock edition
Linked issues
Comments

you can not place blocks in the nether roof in bedrock edition
BabiNatiGech133
(Unassigned)
Unconfirmed
Windows
minecraft pe bedrock every version every device
1.21.90.25 Preview, 1.21.90.23 Preview, 1.21.80
Retrieved
This is intentional, see MCPE-18263.