The mana cost of a rune ritual requires you to have more mana on you than required.
Essentially when you have a recipe that needs 500.000 mana and you have a filled band of mana on you (the first botania one) the recipe says that you don't have enough mana on you. When you change the recipe to take 499.999 mana the recipe works and you'll have 1 mana left in the band
The same happens if a recipe requires 2.000.000 mana and you have a filled greater band of mana or with 4.000.000 mana and a greatest band of mana.
Using:
Minecraft 1.16.5
Forge 36.1.32
MythicBotany-1.16.4-1.3.15
Botania-1.16.5-418

This is an issue with botania. I'll open a pull request for it there.

Should be fixed now, that the botania pull request is merged.