I have been trying to buy tickets for the Jeronimos Monastery in Belem but it seems they no longer sell them online.
Or have I missed the online site where I could purchase them? I am thinking that I may do the Lisboa Card which gives free
admission to lots of museums and transportation. Does anyone have experience with that card? Is it worthwhile?
Can you avoid the lines if you have the card?
Thanks in advance for any info you can provide!

Bringing this to the top as I am interested in an answer, also.

Me too.

losaltos,
You're correct that online sales of Jerónimos tickets have been discontinued (for quite some time).

I wouldn't bother purchasing the Lisboa Card just to use it for entrance to the Jerónimos Monastery, since you can purchase Monastery tickets at the Tourist Office booth in the park directly across from the Monastery.
You can also purchase the Jerónimos tickets inside the Archaeological Museum directly to the west of the Monastery, at the ticket office there or at the ticket machine inside.
We've never purchased the Lisboa Card, but to see if it would actually be a money saver for you, you could make a list of the monuments you intend to visit and their entrance fees and compare the final tab with the price of the card.

Lisbon public transportation is cheap. If you have an extended stay and know that you will be using the metro and/or the commuter rail to travel to Sintra and/or Cascais, you can opt for the Via Viagem transportation card and charge it with money (called "zapping").
And taxis are also inexpensive. Uber does function in Lisbon as well.

Read about the Viva Viagem card here to decide if it will be useful to you-
https://www.metrolisboa.pt/en/buy/viva-viagem-card/

https://www.lisbonguru.com/viva-viag...m-ferry-train/

"You can also purchase the Jerónimos tickets inside the Archaeological Museum directly to the west of the Monastery, at the ticket office there or at the ticket machine inside."

Yes do this, don't stand in the horrendous line for the Monastery, whatever you do.

Use Uber to get to the Monastery, the transit options get packed. And it gets charged to your regular credit card, no worries at all. The Uber and taxi drivers get along in Lisbon, plenty of work for everyone.

Ditto to what mlgb has told you about the Jerónimos lines. Tickets are not required to visit the church, only the Monastery.
And I also would avoid using the always packed and pickpocket infested tram 15 to reach the Belém monuments.