I forgot to report a 1099-R in 2018, shall I report it this year?

NO!  Each year is separate.  You need to amend your 2018 return to add it.  Or do you need to file an original 2018 return?

Amend 2018

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579

No you cannot just stick a 2018 tax document on your 2019 tax return.  That is not permitted.  You need to amend your 2018 return to enter it.

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed?jump_to=answer_3288565

A 1040X cannot be e-filed.  It must be printed and mailed.  It will take about 4 months for the IRS to process an amended return.  It is a good idea to use a mailing service that will track it, like UPS or certified mail, so you will know it was received.

Wait about 3 weeks from mailing and you can check the status of your amended return:

https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1

https://www.irs.gov/filing/individuals/amended-returns-form-1040-x/amended-return-frequently-asked-questions

https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend