@WoollierTiara12 What number of total slots were trying to achieve? In terms of upslotting:
As @Jamesmor stated, you should affix both an S4 and S5 onto your Rivalate while upslotting because these won't face the upslotting penalty and will therefore increase your overall rate of success. Your Rivalate will have an S5 automatically available when affixing if you enhance it to +35 beforehand. Enhancing to +35 unlocks a weapon's Augment Factor (SAF) and every Rivalate has an S5 as its SAF. There are mutliple sources for S4s, but any Atlas weapon will have one as its SAF or you could purchase S4: Escalating Pursuit from the Rising Weapon Badge 5 shop or the Player Shop. If you want to move an S4 from another weapon onto your Rivalate you can use the Transfer S-Grade Augments menu option at Monica/Dudu.
As Jamesmor also stated, when upslotting you should only select "junk augments" that have a 100% chance of succeeding prior to getting hit with the upslotting penalty. These are augments like Might 1, Precision 1, Stamina 1, Spirit 1, Toughness 1, Reflex 1, any of the Ward 1s, any Fever, etc.
You'll also want to purchase an Augmentation Aid item from the Ex-Cube exchange vendor on the 2nd floor of the shopping plaza. You can buy a 30% AA item for 20 cubes and a 40% AA item for 100 cubes. These items will increase the chance of success for every augment by their stated percentage and are great for counter-acting the upslotting penalty. So long as you have this item in your inventory you'll be given a chance to use it once you select all of your affixes.
Because it's not an affixing boost week, the highest number of slots you can get to with 100% success using the tips above and a 40% AA is 5-slots. Trying to get to 6-slots will leave you with a 95% chance of success on your non-S-grade augments. So if you select an S4, S5, and 4 junk augments, those 4 junk augments will have a 95% chance of success while the S4 and S5 will still have 100% chance (no upslotting penalty). However, keep in mind that you need to succeed on all 4 of those 95%s to upslot to 6-slots so your overall chance of success is actually 81.45% (i.e. 0.95^4).
If you want to get to 6-slots with 100% chance of success, you'll need to do an extra step once you upslot your Rivalate to 5 slots. In particular, you'll need to affix an augment called "Augment Guidance" onto your 5-slot Rivalate because this will give all of your affixes an extra 5% chance of success when trying to get to 6-slots. To get Augment Guidance onto your 5s Rivalate you'll need three 5s fodder weapons that already have Augment Guidance on them (can be purchased from Player Shops). With 3 copies of Augment Guidance you'll have an 80% chance of moving it over to your Rivalate so with a 30% AA item this will go to 100%.
Going up to 7 or 8 slots will always entail some sort of risk, even with Augment Guidance and with using the rare augmentation aid items like a 45% or 50% AA. Unless you don't mind spending a lot of resources re-upslotting when you fail, it's best to wait for an Affix Boost week to try for these upslots.