I've noticed time and again cases where rules for a calculated field do not provide options to select all of the fields availble in a lookup for an action rule.
And manually typing the field name does not pass the validation check (it says the field does not exist) - but sure enough it is there. This is not an access/permissions issue, nor an issue with the field type (I've check) and other fields on the entity lookup'd with the same type are avaialble and selectable.
I've noticed that it appears to be somewhat random what shows up when in some cases. This afternoon I continually tried to create a rule with a certain field and it was not there in the list of options. I came back and hour later, and sure enough now it is available, but one of the other fields previously available for selection was not...
No changes to those fields in the entity was made (ie: I didn't add or change them and perhaps have to wait for the update to occur - those values and that lookup'd entity hadn't changed in weeks and PowerApps and everything else is able to access it).