ALTER TABLE SWITCH statement failed. SWITCH is not allowed because source table '' contains primary key for constraint ''
Correct. You can't do a switch out if there's another table which is dependent on the switching table with a FK. 

Imagine that you have PK values 1,2,3, and a foreign key into it. Now you want to switch out value 3, so it'll have to check that 3 isn't mentioned in the FK table - that's a large amount of work, and a lot more than the metadata operation that partition switching is meant to be.

...so it's not allowed. Do it without FKs.